Scanned documents are almost always the easiest PDFs to shrink, because each page is a photograph of paper — mostly flat, mostly plain, and full of the kind of redundancy that compression is good at removing. The catch is that the scanning step itself often bakes in far more resolution than the final use needs, so you end up compressing away detail you never wanted in the first place.
This guide covers both sides: fixing a scan you already have with the PDF Compressor, and scanning in a way that avoids the problem next time.
Fixing a scan you already have
- Open the PDF Compressor and upload the file.
- Choose a medium or strong compression level — scanned documents tolerate strong compression better than photographs, because the content is simpler.
- Download and check the result at normal reading size, not zoomed in.
- If any page still looks unclear, that page may have been scanned at an unusually low quality to begin with; re-scan just that page if possible.
Why scans start so large
Most scanner software defaults to 300 DPI, sometimes 600 DPI, because that is the standard for print-quality archiving. For a document that will only ever be viewed on a screen — which is most uploads and emails — that resolution is far beyond what is needed. A screen shows roughly 72–150 DPI worth of visible detail depending on the device, so anything scanned well above that is carrying detail nobody will ever see, at a real cost in file size.
Scanning settings that avoid the problem
- Resolution: 150–200 DPI for anything that will be read on screen. Reserve 300 DPI for documents you specifically need to print at high quality later.
- Colour mode: use greyscale or black-and-white for text documents instead of colour. A black-and-white scan can be a tenth the size of the same page scanned in colour, with no loss of legibility for plain text.
- File format: if your scanner app offers a choice, JPG compression within the PDF is usually smaller than an uncompressed or lightly compressed default.
Getting these right at scan time often means you barely need to compress afterwards at all.
Multi-page scans
When scanning a multi-page document, size adds up fast — a 20-page contract at 300 DPI colour can be 30–60 MB before you have touched it. If your scanning app lets you set resolution and colour mode per batch, apply the lighter settings from the start rather than scanning everything at maximum quality and compressing the whole stack afterwards. If you have already scanned a large batch at high settings and only need to send part of it, Split PDF first, then compress just the pages you are keeping — it is faster and gives a better result than compressing the whole document and discarding most of it.
What the recipient actually needs
Before scanning or compressing, it is worth pausing on what the document is actually for. A file going into a permanent legal or medical archive genuinely benefits from a higher-quality, larger master copy kept safely on your own drive. A file being uploaded to a portal, attached to an email, or sent over chat almost never needs that same quality — the recipient is reading it on a screen once, not archiving it for decades. Keeping one high-quality master and generating a smaller working copy for day-to-day sending avoids re-scanning the same physical document twice, once at each quality level.
Photographed “scans” from a phone
A document photographed with a phone camera behaves differently from a real scanner: it usually has uneven lighting, a slight skew, and background outside the page edges, all of which add detail a proper scan would not have. Cropping tightly to the page and correcting the angle before compressing — most phone scanning apps do this automatically — removes a surprising amount of unnecessary size on top of what compression alone achieves. If you are regularly scanning documents this way, it is worth reading through how to get the framing and lighting right, since it affects every scan afterwards, not just the current file.
Frequently asked questions
Is 150 DPI really enough to read comfortably on screen?
Yes, for standard text documents. It is noticeably lower than print resolution but still sharp on any modern screen at normal reading size.
Should I scan in colour if the original document is colour?
If colour genuinely carries information — a diagram, a stamp, a photograph within the page — keep it. If the document is text with the odd coloured logo, greyscale usually loses nothing that matters.
Does a lower DPI scan look worse when printed later?
It can, which is why archival copies you may need to print at full quality are worth keeping at 300 DPI separately from the smaller version you send around day to day.
Can I fix a scan that is already too small and blurry?
Not by re-compressing — compression cannot restore detail that scanning never captured. The only real fix is re-scanning at a slightly higher setting.
Is the tool free and private?
Yes. No sign-up, no watermark, and files are removed from the server automatically about an hour after processing.