Convert multiple PNGs to one PDF — page size, margins, lossless embed

The short version

1. Drop your PNGs on the tool. Select the whole folder — the order is preserved.
2. Pick Letter (US) or A4 (everywhere else). Leave it on Auto if pages should match image size exactly — best for screenshots.
3. Leave Fit-to-page with 12mm margins on unless you have a specific layout in mind.
4. Drag to reorder pages if needed.
5. Download. One PDF, alpha channels preserved, pixel-perfect.

If that's enough, stop here. The rest of this post covers the decisions that actually change the output: page size, margins, the lossless-vs-compressed trade-off (our default changed recently), transparency handling, DPI for print, and why your 10 MB of PNGs somehow becomes either a 3 MB PDF or a 45 MB PDF depending on the tool.

Page size — Auto vs Letter vs A4

This is the first real decision and most "png to pdf" tutorials skip it because there's no universally right answer.

• Auto (page matches image) — best for screenshots, UI mockups, scroll-capture pages, any workflow where the image is the page. No white bars, no forced scaling. Use this by default if the PDF is for digital reading, not printing.
• Letter (8.5 × 11 in) — US default for printing. Use this if the PDF is going to a US printer, or if you're submitting to a form that expects Letter. Images are scaled to fit with margins.
• A4 (210 × 297 mm) — everywhere else in the world. Use this for European / Asian / Latin American / African destinations.
• Legal / Tabloid / custom — rare, for CAD plots, receipts longer than 11", or business-card layouts.

Warning: if you pick Letter and your PNG is taller than the page at the target DPI, the tool either scales it down (reducing effective resolution) or splits it across pages. We default to scale-to-fit because split-across-pages breaks text. If you need a very tall screenshot on one page, pick Auto.

The lossless change (and why your PDF just got 3-6× bigger)

Until about a week ago, our PNG-to-PDF was doing what most tools do: extracting each PNG's pixels, re-encoding to JPG at quality 85, and embedding the JPG in the PDF. That produces tiny PDFs — 500 KB for 10 screenshots is realistic — but it quietly applies JPG compression to files the user chose PNG for. Screenshots, UI mocks, line-art, diagrams — exactly the material that pays the biggest penalty for JPG. Text gets ringing artefacts around edges. Sharp colour transitions blur. Alpha channels get flattened against whatever background colour the encoder guessed.

Our default is lossless PNG embedding. Each PNG becomes a real /FlateDecode stream inside the PDF — the same bytes that were in the original PNG, embedded 1:1. Alpha channels ride along via the PDF's SMask mechanism, so transparency is preserved. The PDF is pixel-for-pixel identical to the source on any viewer that can render PDFs (which is everyone).

The honest trade-off:

For most workflows — documentation, design reviews, scanned receipts, screenshot bundles — the quality gain is worth the size cost. If file size is the constraint (emailing to a 25 MB Gmail cap), we still surface a "JPG-compressed" toggle that reverts to the old behaviour, and you can always follow up with our PDF compressor for another 20-40% off losslessly.

Margins and fit — what the four options actually do

Standard PDF builders offer four fit modes. We use the same names:

• Fit (default) — image scaled so it fits inside margins, preserving aspect ratio. Safe; some whitespace on one axis unless the image is already letter-proportioned.
• Fill — image scaled and cropped to fill the page edges. Good for full-bleed photo books; bad for screenshots (edges cut off).
• Stretch — image warped to fill the page, breaking aspect ratio. Almost never what you want. We warn when you pick it.
• Original size — image placed at its pixel size translated through a DPI (we default to 72 DPI for screens, 300 for print). Use this when the original pixel resolution matters, e.g. preserving 300 DPI prints.

Margins default to 12 mm (≈0.47 in) — slightly tighter than Word's default 1" so more of the image is visible, but still printable on any consumer printer. You can set 0 mm margins for borderless, or go wider if you're printing onto letterhead.

Order of pages — drag-to-reorder, filename sort, or EXIF date

This is the "oh no" moment of PNG-to-PDF. You dropped 30 screenshots, the tool sorted them by filename, and filename sorting does not match Windows' visual order. Screenshot 10.png sorts before Screenshot 2.png because character-by-character string comparison puts "1" before "2". You get pages in the wrong order.

We sort with natural sort by default — the human way: 1, 2, 3, ..., 9, 10, 11 — matching what you see in File Explorer and Finder. If that still isn't right, drag-to- reorder the thumbnail strip before you convert. Alternative sort modes:

• Natural (default) — human-friendly numbering.
• Alphabetic — strict character-by-character.
• Date modified — from file system timestamps, useful when filenames are random hashes.
• EXIF timestamp — from image metadata, ideal for photo sequences.
• Manual — drag thumbnails yourself.

Transparency — what happens to your PNG's alpha channel

PNG supports 8-bit alpha channels (256 levels of transparency); PDF supports them too, via SMask. The paths diverge by tool:

• Our tool (lossless default) — alpha preserved exactly. A PNG with transparent background produces a PDF page with transparent background. Open in Acrobat on a dark UI, you see the transparency; on a light UI, you see through to the page.
• Our tool (JPG-compressed mode) — alpha flattened to white, because JPG has no alpha.
• Windows Photos "Print to PDF" — alpha flattened to white; re-encodes to JPG internally.
• Smallpdf / iLovePDF — alpha flattened to white in most cases; some now preserve it on paid tiers.
• Adobe Acrobat — preserves alpha, but the output is larger than ours because it also embeds an ICC profile by default.

Transparency in a PDF is useful for overlays, watermarks on presentations, annotation comps — the use-cases where people pick PNG in the first place. If you're just printing screenshots, flattening to white is fine and saves some bytes.

DPI for print — the bit nobody explains

If the PDF is going to a printer, the image DPI matters. Rule of thumb:

• 72-96 DPI — screen-only. Will look pixelated printed.
• 150 DPI — okay for office printers, draft docs.
• 300 DPI — commercial print quality (books, posters, pro photo).
• 600 DPI — fine-art and archival.

A 1920×1080 PNG at 300 DPI is 6.4" × 3.6" on paper; at 72 DPI it's 26.7" × 15". Same bytes, different implied physical size. The "DPI" field in our tool sets the implied size; it doesn't resample the image.

If your source PNGs are too low-resolution to print clean at 300 DPI, upscale them first through our AI upscaler (2× or 4×, neural, free). Going from 600 px wide → 1200 px wide via bicubic alone loses detail; neural upscaling usually does better.

File size — why yours is 45 MB (and how to cut it)

Common scenario: 10 full-resolution screenshots of a web app, each 2000×1400 px, each around 300-500 KB as PNG. Through different tools:

• Old JPG-compressed PDF (q=85) — ~1.2 MB total. Blurred text, JPG artefacts around UI edges.
• Our lossless default — ~4-6 MB total. Pixel-perfect. This is the new default.
• Windows Photos "Print to PDF" — ~8-15 MB. Re-encodes at variable quality.
• Adobe Acrobat (defaults) — ~10-20 MB. Embeds ICC profile + other metadata.
• macOS Preview "Export as PDF" — ~3-5 MB. Similar approach to ours.
• Smallpdf (free) — ~2-4 MB. JPG-compressed, watermark added.

If you need under a specific size cap — Gmail 25 MB, most web forms 10 MB — either (a) export with our JPG-compressed toggle (revert to old behaviour) or (b) export lossless and run through our PDF compressor, which preserves quality while re-encoding images more aggressively than the PNG-to-PDF step. The second option is usually better because the compressor can make per-image decisions about which pages tolerate JPG and which need to stay PNG.

The honest compare

Summary: if you're on macOS with <10 images, Preview is fine. If you're on Windows or doing a real batch, our lossless in-browser converter is the shortest path. If you're scripting a document pipeline, ImageMagick. Adobe Acrobat only if you need PDF/A compliance or scripted form integration.

Common workflows

Documentation bundle from screenshots

Take screenshots with Windows' Snipping Tool, number them in order (01-login.png, 02-dashboard.png, ...), drag the folder onto our tool, set page size to Auto, leave natural sort on, export. Result: a clean documentation PDF with one screenshot per page at native resolution.

Scanned receipts for tax season

Scan each receipt as PNG (or convert from JPG to PNG if your scanner outputs JPG). Drop on the tool. Pick Letter or A4 to match your printer, leave margins at 12 mm, export. Run through our PDF compressor to get under email caps while keeping receipts readable.

Design review PDF

Export mockups from Figma as PNG, drop on the tool, set page size to Auto so the PDF pages match the mockup dimensions exactly. Lossless default preserves every pixel including sub-pixel text edges your reviewer will notice.

Photo book (print)

Prepare photos at 300 DPI in their final pixel dimensions. Drop on the tool. Set page size to match your print dimensions (often custom: 8×8" for square photo books). Set DPI field to 300 so the printer reads page size correctly. If photos are large, consider toggling JPG-compressed mode — at q=95 the quality loss is invisible on print, and the file is a fifth the size.

Works well / doesn't work

Works well

• Screenshot bundles — pixel-perfect with the new lossless default.
• Mixed-size PNG collections — per-page size via Auto mode.
• PNGs with transparent backgrounds — alpha preserved via SMask.
• Batches up to ~300 images on a modern laptop.
• Privacy-sensitive files — conversion runs entirely in your browser.
• Offline use after first load (PWA-cached).

Doesn't work (well) yet

• PDF/A archival compliance — on the roadmap; use Acrobat if you need it today.
• Embedded hyperlinks per page — PDF bookmarks from filenames, yes; hotspots inside images, not yet.
• Very long documents (500+ pages) — memory ceiling on browsers; split in batches.
• Password-encrypted output — planned; use a separate step for now.
• Animated PNG (APNG) — we take the first frame only.

Common questions

Will my PDF preserve transparency?

With the lossless default, yes — alpha channels ride through as SMask entries in the PDF and render correctly in Acrobat, Preview, Chrome, Edge, and Firefox. With the JPG-compressed fallback toggle, no — alpha gets flattened to white, because JPG has no alpha channel.

Why is the PDF bigger than I expected?

Because the default is lossless. Tools that re-encode every PNG to JPG quality 85 before embedding produce smaller files at compressed quality. We embed PNG data directly: pixel-perfect, about 3-6× larger. If size matters more than fidelity, enable the "JPG-compressed" toggle in advanced settings.

Can I reorder pages after dropping?

Yes. The tool shows thumbnails; drag to reorder. Alternatively pick one of the sort modes (natural, alphabetic, date modified, EXIF timestamp).

What's the file cap?

On free-tier, ~300 images per session is comfortable on a modern laptop. This is a browser-memory ceiling, not a server-imposed cap. For larger documents, split into two PDFs and merge with our PDF merger.

Will my files upload anywhere?

No. All processing runs in your browser via WebAssembly. The PNGs never leave your machine; we never see them; no analytics read their contents.

Can I add text to pages?

Not in the PNG-to-PDF tool itself — it's for assembling images into a PDF, nothing else. For text overlays you'd need a PDF editor. That's on our roadmap; for now pair the result with a free viewer like Foxit or Acrobat Reader's free comment tool.

What's the difference between PNG-to-PDF and JPG-to-PDF?

Mostly the input. A JPG is already lossy, so JPG to PDF re-embeds the same JPG bytes losslessly (nothing to gain from re-encoding). PNG-to-PDF has the choice: embed losslessly (our new default) or re-encode to JPG for size. If your source is PNG specifically because you need precision, you want the lossless path.

Ready?

PNG to PDF →. Drop the folder, pick Auto for screenshots or Letter/A4 for documents, download the PDF. Free, in your browser, no upload, no watermark, no sign-up. Alpha channels preserved, pixel-perfect output. If size is an issue, chain through our PDF compressor; for JPG sources, try JPG to PDF instead.