Compress a scanned PDF
A ten-page scan can easily weigh 40 MB while a ten-page word-processed document weighs 200 KB. The difference is not the number of pages — it is that a scan stores each page as a full-resolution image rather than as text.
That also makes scans the best candidates for compression: they are where the biggest, safest reductions live.
Why scans are so large
Scanners default to settings built for archival fidelity, not for sharing. A page scanned in colour at 600 dpi contains roughly nine times as much data as the same page at 200 dpi, and colour data itself triples the size of a page that only contains black text.
- Resolution: 600 dpi is for reproduction; 200–300 dpi is plenty for reading and printing text.
- Colour mode: greyscale or black-and-white removes most of the weight from text pages.
- Format: pages stored as lossless images do not shrink until they are re-encoded.
How much you can remove safely
For a text scan, Medium compression typically removes most of the excess with no practical loss of legibility. Maximum goes further and is the right choice when a portal enforces a small limit, at the cost of softer edges on small print.
Scans of photographs, signatures with fine pen detail, or medical images deserve more caution — compress a copy and compare before discarding the original.
Fix it at the scanner when you can
If you are producing the scan yourself, changing two settings beats any amount of post-processing: scan in greyscale, and set 300 dpi. The resulting PDF is often small enough that no compression is needed at all.
Step by step
- 1Open the Compress tool and add your scanned PDF.
- 2Choose Medium for a readable, much smaller file.
- 3Open the result and check the smallest text on the busiest page.
- 4If you need it smaller still, run Maximum on the original scan.
- 5Keep your original scan if the document may need to be reprinted at full quality.
Frequently asked questions
- Why is my scanned PDF so big?
- Each page is stored as an image. High resolution and colour scanning multiply the data for every single page, so size grows quickly with page count.
- Can compression make scanned text searchable?
- No. Compression changes image quality, not content. Making a scan searchable requires OCR, which AllDayPDF does not perform.
- How much smaller can a scan get?
- Reductions of 70–90% are common for colour text scans, because most of the original data carries no useful information for reading.
- Is compressing a scan free?
- Small and Medium compression are free. Maximum compression is included with Pro.
Free, in your browser, no account
Open the tool and get it done in a few seconds.