The biggest saving in our test came from a file where no picture was touched. The 226-page C2PA technical specification went from 10.0 MB to 2.95 MB, and the compression level made no difference at all: Light, Recommended and Strong gave the same file to the byte. Of its 261 internal streams, 230 had been written with no compression at all, 9.2 MB of the file stored as it was.
That file is the exception. For most PDFs the answer to “how much can I compress this” depends on one thing: how much of the file is photographs. We ran eight PDFs through our Compress PDF tool at all three levels on 24 September 2026 to put numbers on it.
Eight PDFs at three levels
Two we built with our own JPG to PDF tool: three large JPEG photos, one per page, and a stand-in for a scan, made by turning three pages of a NIST report into 300 dpi JPGs and putting them back into a PDF, which is what a phone scanner app hands you. The other six are public PDFs: two NIST reports (one from Word, one from LaTeX), an arXiv paper, the C2PA specification, the IRS W-9 form, and a one-page memo printed from Chromium.
| Original | Light | Recommended | Strong | |
|---|---|---|---|---|
| Three photos, one per page | 4.83 MB | 16.0% | 10.7% | 8.2% |
| Three scanned text pages, 300 dpi | 1.82 MB | 37.8% | 17.0% | 8.3% |
| NIST AI framework, LaTeX, 48 pages | 1.95 MB | 69.5% | 67.2% | 64.8% |
| Attention Is All You Need, 15 pages | 2.22 MB | 68.3% | 68.3% | 68.3% |
| C2PA specification, 226 pages | 10.0 MB | 29.4% | 29.4% | 29.4% |
| NIST Cybersecurity Framework, Word, 32 pages | 1.52 MB | 92.7% | 87.2% | 80.0% |
| IRS Form W-9, 6 pages | 141 KB | 92.2% | 92.2% | 92.2% |
| One-page memo printed from Chromium | 42 KB | 93.1% | 93.1% | 93.1% |
Photos and scans are where compression pays: 83 to 89% off at Recommended and 92% at Strong, and the level you pick moves the result a long way. Everything else falls into two groups. Some files shrink by a third without a single picture changing, because the tool also packs the file’s internal objects more tightly, compresses streams that were stored raw and drops objects nothing points to. The arXiv paper lost 32% that way. Its three pictures were all left as they were; none of them met the conditions our tool sets before it replaces a picture. The rest, forms and short text documents, lose 7 or 8%, most of it metadata.
If your PDF is a report with text and a few charts, the level barely matters. The NIST framework from Word has 13 pictures; the tool re-saved three at Light and four at the other levels, and the whole range from Light to Strong was 13 percentage points. Picking Strong there only softens the few photos it has.
We think the default most PDF tools choose for you, a single “compress” button, hides this. On a scan the difference between the lightest and strongest setting was 4.5 times in file size; on the IRS form it was nothing. Knowing which kind of file you hold tells you whether to spend time on the setting.
What Strong does to a scanned page
We rendered the same paragraph of the scanned test page at 200% zoom from each version. Light is hard to tell from the original. Recommended is slightly soft around the letters. Strong, at 100 dpi, is still easy to read, but each letter has a faint fuzz around it and a blue heading turned blotchy. For email and upload portals that cap attachments, Strong is fine. For anything that will be printed or zoomed into, like a signed contract or a form with small print, stay on Recommended: the scan was 308 KB there, already a sixth of the original.
Scanned pages took the longest. Light spent 2 seconds on three pages on a fast laptop, mostly decoding and re-encoding 300 dpi pictures; the photo PDF took under a second at every level.
When a PDF still won’t fit
If a text PDF won’t go under an upload limit, compression isn’t the lever. Split the pages you need to send into a smaller file with Split PDF, and check which pages carry the pictures first: in our test, five pages of the NIST framework with its cover photo came out at 509 KB, and five pages of plain text from its middle at 137 KB. Details are in what survives a merge or split.