How much smaller can a PDF get? We compressed 14 real documents to find out
Short answer: It depends on what kind of PDF it is, not on how big it is. In our test of 14 public-domain documents, colour scans shrank by 39–86% at the default Balanced level and by up to 98% at Strong. Black-and-white scans, long text documents and e-books from professional publishing software barely changed. Every compressed file kept every page, and its text read back exactly as before.
Updated
The results in brief
39–86%
Saved on colour scans at Balanced, the default level
Up to 98%
Saved on colour scans at Strong
About 1%
Saved on black-and-white scans, at any level
39 of 39
Results that kept every page and every word
How a PDF was made decides the saving, not how big it is. A 10.1 MB scan of a single page came down to 1.5 MB. An 11.3 MB NASA e-book shrank by just 1.3%. This page shows every result and explains the difference, so you can tell in advance what to expect from your own file.
How we tested
We chose 14 documents covering the kinds of PDF people most often need to shrink: colour scans, black-and-white scans, e-books full of photographs, slide decks, and text documents. Every one is in the public domain — published by the US government, or before 1927 — so anyone can download the same files and repeat the test.
- Each document went through Compress PDF at all three levels, Light, Balanced and Strong, using the same code and the same limits as a job on this site.
- After every run we checked that the result was still the same document: the same number of pages, and the text on every page reading back exactly as it did from the original.
- We looked at the most heavily compressed pages at 150% zoom to judge their quality for ourselves.
- Sizes are in megabytes of 1,000,000 bytes.
How much smaller each document got
Colour scans
Senate Joint Resolution 116, 1941
1 page · 10.1 MB
Light75%Balanced86%Strong98%
Senate Bill 3060, 1930
8 pages · 8.9 MB
Light0.2%Balanced50%Strong90%
Clara Barton's Red Cross petition
12 pages · 9.7 MB
Light0.1%Balanced39%Strong86%
Ridgway's Color Standards, 1912
174 pages · 52.5 MB
Refused: too much image data for one job
E-books full of photographs
The Saturn System (NASA)
111 pages · 30.0 MB
Light19%Balanced58%Strong77%
Hubble Focus: Galaxies (NASA)
45 pages · 4.1 MB
Light4.5%Balanced4.5%Strong12%
Earth as Art (NASA)
172 pages · 11.3 MB
Light1.3%Balanced1.3%Strong1.3%
Slide decks
NASA Goddard project slides
18 pages · 4.0 MB
Light0.3%Balanced0.3%Strong47%
NASA telescope study slides
20 pages · 1.3 MB
Light5.3%Balanced7.1%Strong36%
Text documents
IRS Form 1040
2 pages · 0.22 MB
Light29%Balanced29%Strong29%
IRS Publication 17
142 pages · 3.1 MB
Light0%Balanced0%Strong0.2%
Black-and-white scans
NASA report on the NACA library, 1999
15 pages · 0.96 MB
Light1.4%Balanced1.4%Strong1.4%
NACA Report 460, 1933
61 pages · 6.1 MB
Light0.8%Balanced0.8%Strong0.8%
Federal Reserve Bulletin, 1915
78 pages · 7.4 MB
Light0.9%Balanced0.9%Strong0.9%
Each bar shows all three levels at once. The darkest part is Light, the middle shade Balanced, and the palest Strong; where a bar has only one shade, all three levels gave the same result.
Colour scans: the biggest savings
A scanner stores each page as a photograph, usually at 300–400 DPI and at high quality. That is several times more detail than a screen can show, and it is exactly what compression removes. The three National Archives scans we tested shrank by 39–86% at Balanced and by 86–98% at Strong. The single-page 1941 resolution went from 10.1 MB to 1.5 MB at Balanced, and to 0.23 MB at Strong.
Light barely touched the two scans made at 400 DPI, saving 0.1–0.2%. That is by design: Light keeps images at print quality. It still took 75% off the 1941 resolution, because that page had been saved at an unusually high quality setting.
At 150% zoom we could not tell the Balanced pages from the originals. The Strong pages are visibly softer, and faint marks in the paper fade, but the typed text stays easy to read on screen. If the document will be printed, use Balanced rather than Strong.
One colour scan was refused: a 174-page book from 1912, 52.5 MB. Its page images add up to more than one job is allowed to process. A document like that has to be split into parts first, and each part compressed separately.
Black-and-white scans: already as small as they get
Older archive scans, and office scanners set to black and white, store each page as a 1-bit image: every dot is either black or white, packed with fax-style compression. A page like that takes 60–100 KB. All three we tested lost about 1% at every level, because there was nothing left to remove.
Compress PDF leaves these images alone on purpose. Turning them into photographs (JPEG) would make the file bigger, and the text grey and fuzzy.
Long black-and-white scans go through quickly: the 78-page Federal Reserve Bulletin from 1915 holds more than 500 million pixels, but fax-compressed pages are fast to read, and the tool allows for that. If a black-and-white scan is too big to send, though, compression will not help. Split it into smaller parts instead.
E-books and slide decks: it depends how they were made
The three NASA e-books show how much the software that made a PDF matters. Earth as Art and Hubble Focus were made with professional publishing software, which had already sized their photographs for the screen and compressed them well. They shrank by 1.3% and 4.5% at Balanced, and Strong did little more.
The Saturn System was exported from a Mac's print dialog, with some photographs left at around 300 DPI. It came down from 30.0 MB to 12.8 MB at Balanced, and to 6.9 MB at Strong.
The two slide decks, both exported from PowerPoint, saved 0–7% at Light and Balanced, and 36–47% at Strong. Their images were mostly 150–200 DPI. That is too little for Balanced to reduce, which only resizes images above about 200 DPI, but well above Strong's screen-sized target.
Text documents: small to begin with
IRS Publication 17 is 142 pages of text and tables in 3.1 MB. It came back unchanged at Light and Balanced, because nothing could be made smaller, and lost just 0.2% at Strong. Text and fonts are compact already, and the whole document holds a single image.
The two-page Form 1040 shrank by 29% at every level, without containing a single image. That saving came from tidying the file's internal structure: merging objects that were repeated, and packing the rest more tightly. All 199 of its form fields were still there, and still fillable, afterwards.
Full results
| Document | Pages | Original | Light | Balanced | Strong |
|---|---|---|---|---|---|
| Senate Joint Resolution 116 (colour scan) | 1 | 10.13 MB | 2.49 MB (−75%) | 1.47 MB (−86%) | 0.23 MB (−98%) |
| Senate Bill 3060 (colour scan) | 8 | 8.88 MB | 8.86 MB (−0.2%) | 4.46 MB (−50%) | 0.93 MB (−90%) |
| Clara Barton's petition (colour scan) | 12 | 9.71 MB | 9.70 MB (−0.1%) | 5.93 MB (−39%) | 1.33 MB (−86%) |
| Ridgway's Color Standards (colour scan) | 174 | 52.55 MB | Refused | Refused | Refused |
| The Saturn System (e-book) | 111 | 30.04 MB | 24.35 MB (−19%) | 12.75 MB (−58%) | 6.85 MB (−77%) |
| Hubble Focus (e-book) | 45 | 4.10 MB | 3.92 MB (−4.5%) | 3.92 MB (−4.5%) | 3.62 MB (−12%) |
| Earth as Art (e-book) | 172 | 11.31 MB | 11.17 MB (−1.3%) | 11.17 MB (−1.3%) | 11.17 MB (−1.3%) |
| NASA Goddard slides | 18 | 4.04 MB | 4.03 MB (−0.3%) | 4.03 MB (−0.3%) | 2.14 MB (−47%) |
| NASA telescope study slides | 20 | 1.27 MB | 1.21 MB (−5.3%) | 1.18 MB (−7.1%) | 0.82 MB (−36%) |
| IRS Form 1040 | 2 | 0.22 MB | 0.16 MB (−29%) | 0.16 MB (−29%) | 0.16 MB (−29%) |
| IRS Publication 17 | 142 | 3.09 MB | Unchanged | Unchanged | 3.08 MB (−0.2%) |
| NASA report on the NACA library (b/w scan) | 15 | 0.96 MB | 0.94 MB (−1.4%) | 0.94 MB (−1.4%) | 0.94 MB (−1.4%) |
| NACA Report 460 (b/w scan) | 61 | 6.13 MB | 6.08 MB (−0.8%) | 6.08 MB (−0.8%) | 6.08 MB (−0.8%) |
| Federal Reserve Bulletin, 1915 (b/w scan) | 78 | 7.36 MB | 7.29 MB (−0.9%) | 7.29 MB (−0.9%) | 7.29 MB (−0.9%) |
Where the documents came from: the US Internal Revenue Service (Form 1040 and Publication 17); NASA (the three e-books); the NASA Technical Reports Server (the two slide decks, NACA Report 460 and the 1999 report); the US National Archives, through Wikimedia Commons (the three colour scans of Senate records); FRASER at the Federal Reserve Bank of St. Louis (the 1915 bulletin); and the Internet Archive (Ridgway's Color Standards and Color Nomenclature). The IRS revises its documents every year; we tested the versions available on 2 October 2026.
What this means for your PDF
- Start with Balanced. It made a large difference wherever there was something to gain, and we could not see the change at normal zoom.
- Scanned in colour? Expect a large saving. Choose Strong if the file only needs to be read on screen and has to fit a tight limit.
- Scanned in black and white, or mostly text? Compression will change little. If the file is still too big, split it or remove pages nobody needs.
- An e-book or brochure from a publisher may already be optimised. If Balanced saves almost nothing, try Strong once; if that does not help, the file is about as small as it will get.
- Keep your original if the document might be printed. Compression lowers image quality, and that cannot be undone.
For step-by-step instructions and common email and upload limits, read how to reduce a PDF's file size.