PDF compression, measured on a scan and a text file

"Compress PDF" means different things depending on what is inside the file, and most sites do not say which. This page reports what this site's compressor did to two documents on 17 September 2026, with the settings and timings, so you can predict what it will do to yours.

The two test files

A scan-style document. Five A4 pages, each a 2,480 x 3,508 pixel photograph of a page (300 dpi) saved as JPEG at quality 92, the way a phone scanning app saves. Total 14.86 MB.

A text document. Twenty A4 pages of plain text set in Times Roman, the way a word processor exports. Total 21 KB.

Both were run through the Compress PDF tool at each of its three settings in Chrome on a laptop. The times include reading the file and writing the result.

Results on the scanned document

Setting	What it does	Output size	Smaller by	Time
Screen	Draws each page at 72 dpi, JPEG quality 55	245 KB	98 percent	10.3 s
Balanced	Draws each page at 101 dpi, JPEG quality 72	658 KB	96 percent	9.6 s
High	Draws each page at 144 dpi, JPEG quality 85	1.65 MB	89 percent	13.2 s

Every setting made the scan far smaller, because a 300 dpi photograph of a page holds much more detail than anyone reads on a screen. "Balanced" is the default and is readable on a laptop at normal zoom. "High" is for a document that will be printed again. "Screen" is for a file that must fit under a tight email limit and will only ever be read on a phone.

Results on the text document

Setting	Output size	Change
Screen	1.67 MB	80 times larger
Balanced	3.85 MB	183 times larger
High	8.73 MB	415 times larger

This is the result most compression tools hide. A text PDF stores letters as font instructions, which is the most compact way there is to store a page of text. Drawing those pages as images throws that away and replaces each page with a photograph of itself. The file gets larger, and the text stops being selectable.

Because of this measurement, the tool now checks the result. When the output would not be at least 2 percent smaller, the original file is returned unchanged, with a message saying so, and the text stays selectable. You cannot end up with a worse file than you started with.

How to tell which kind of file you have

Open the PDF and try to select a word with the mouse. If the text highlights, it is a text PDF and compression will do little. If the whole page selects as one block, or nothing selects, it is a scan and compression will do a great deal. The file size per page is the other clue: a text page is usually under 50 KB; a scanned page is usually 1 MB to 3 MB.

If the file is a mix

Reports often combine text pages with a few scanned appendices. Compressing the whole file draws the text pages as images too. The better path is to split the file, compress only the scanned part, and merge the two back together. All three steps run on this site without the file leaving your device.

What the settings mean in pixels

The compressor draws pages at a multiple of the PDF's own unit of 72 dpi: 1.0 for Screen, 1.4 for Balanced and 2.0 for High. On an A4 page that is 595 x 841, 833 x 1,178 and 1,190 x 1,683 pixels. The JPEG quality applied to each drawn page is 55, 72 and 85 out of 100. Those two numbers, size and quality, are the whole of what "compression" means here; there is no hidden setting.