How small can a PDF get? Realistic numbers by file type

The floor, by what is on the page

These are measured outputs from our compressor on 11 October 2026, not estimates. The test files: a 10-page colour scan of an A4 lease made at 300 dpi, 5 photos from a 12 megapixel phone camera on A4 pages, and a 500-page text document.

Detail moves these numbers. A second set of 20 test photos with more fine texture came to about 110 KB a photo on the Email preset, not 80, so treat each figure as the middle of a range.

Read it as a lookup table. 8 scanned pages at 72 dpi come to about 480 KB; a 4-page form filled in a word processor plus 2 photographed receipts at the Email preset comes to about 160 KB plus the fonts. Fonts are a one-off cost per file, not per page.

The same numbers answer the portal question. A 100 KB limit fits about 5 scanned pages; a 200 KB limit about 10. Our 10-page scan reached 198 KB at the strongest setting and missed 100 KB, and the tool said so rather than pretend. Cut to 5 pages, the same scan came in at 96 KB.

What sets the floor

A compressor keeps what the eye needs and drops what it will forgive. That is all compression is, and it is why the floor belongs to the content.

Text has almost nothing to drop. A typed page stores the characters and points at a font. It is already tiny, and it must stay exact, so text is never re-encoded. Our 500-page text file is 117 KB and stays 117 KB on every setting.

A scan or a photo has a lot to drop. It is a grid of pixels, and most of them carry detail no reader needs at screen size. Halving the resolution removes 3 of every 4 pixels; lowering JPEG quality simplifies what is left. Each step down costs some detail, and the floor is wherever the detail you need would go too. For a scanned page that is small print: at 50 dpi, a 10-point footnote is a smudge. For a photo, it is whatever the photo is for. A receipt needs its total readable; a portrait needs a face.

Some content is already at its floor. A black and white (1-bit) scan is stored very efficiently from the start, and our compressor leaves it untouched. Vector drawings, such as maps and plans, are kept exactly as they are. Neither shrinks much, which is the subject of why a PDF will not compress.

Typical ratios, and why they vary so much

From the same measurements:

• Colour scan, 10 pages: 17.6 MB to 1.7 MB on the Email preset, about 10 times smaller.
• Phone photos, 5 pages: 14.7 MB to 395 KB on the Email preset, about 37 times smaller.
• Text, 500 pages: 117 KB to 117 KB, no change.

The ratio is wide because it measures how oversized the pictures were, not how good the tool is. A phone photo carries 12 megapixels onto an A4 page that the Email preset stores at about 1 megapixel, so it has the most to lose. A 300 dpi scan has less. A text file has nothing. A file that was compressed once already sits near its floor, and a second pass gains little.

Why 2 tools give different numbers on the same file

Every compressor makes the same 2 choices: the resolution pictures are shrunk to, and the JPEG quality they are saved at. Our presets are Email (110 dpi, quality 65), Web (150 dpi, quality 75) and Print (300 dpi, quality 85), and a size target can go down to 50 dpi at quality 25. Another tool may default lower, and its file will be smaller and softer. Neither number is magic; compare the 2 results by opening them at the zoom your reader will use.

There are 2 more differences worth knowing. Some tools turn every page into a picture, which can shrink a scan further but makes a text page bigger and its text unselectable; ours keeps text as text. And tools count a KB differently: our size targets count 1 KB as 1,000 bytes, the stricter reading, so a file under our 100 KB target is under 100 KB for any portal.

Estimate yours, then test it

1. Count the pages and what is on them: typed text, scans, photos.
2. Multiply by the per-page costs in the table, for the setting you can live with. That is your realistic floor.
3. If the estimate is under your limit, compress with the matching target. If it is over, no setting will get there legibly: send fewer pages or split the file.
4. Check the real number on the result screen, which shows the size before and after.

For a target the portal names, the size pages are set up for you: 100 KB, 200 KB, 500 KB and 1 MB. If you are not sure what is in the file, the inspector shows how much of it is pictures before you start.

The honest limit

Any tool that promises “90 percent smaller, guaranteed” is describing a file full of oversized photos. On our text file the same tool would deliver 0 percent; on a 1-bit fax, close to 0. The ratio belongs to the file. What a compressor can promise is to show you the before and after, keep text sharp, and tell you when a target cannot be met; ours does all 3, and gives you the original back when it cannot make the file smaller. It runs on your device, so nothing is uploaded while you find your floor. If you have no limit to hit at all, what size a PDF should be suggests a sensible one.