Upload limits are arbitrary and unforgiving. Visa application portals commonly cap documents at 1–2 MB. University admission systems often say 500 KB. Gmail refuses attachments over 25 MB. Meanwhile a single phone-scanned page can easily be 4 MB on its own.
The instinct is to slam the compressor to maximum and hope. That reliably produces an illegible document, which gets rejected for a different reason. Here is a better sequence.
Step 0: find out what is heavy
Before compressing, work out which kind of file you have. Select a line of text with your cursor.
- Text highlights → digital PDF. The weight is images, fonts, or nothing at all.
- Nothing highlights → scanned PDF. It is a stack of page images, and image settings are the only lever that matters.
This determines everything that follows, and most people skip it.
Step 1: send fewer pages
The cheapest megabyte is the one you do not send. If the portal asks for your degree certificate and you are uploading a 40-page transcript, extract the two pages that matter. This costs zero quality and is the single most effective step available.
Step 2: fix the resolution, not the quality
For scans, there are two independent dials, and people reach for the wrong one.
- Quality (JPEG compression level) introduces blocking artefacts around letters. Push it too far and text develops a smeared halo.
- Resolution (DPI) reduces the pixel count. Dropping a 600 DPI scan to 200 DPI cuts the data by roughly 89% and remains perfectly readable on screen.
Reduce resolution first, and only then reduce quality. A 200 DPI scan at quality 80 looks dramatically better than a 600 DPI scan at quality 20, at a similar file size.
Useful reference points: 150 DPI is fine for reading on a screen, 200 DPI is a safe general target, 300 DPI is what you want if it will be printed or OCR'd.
Step 3: greyscale, when colour is not information
A black-ink document scanned in full colour stores three channels where one would do. Converting to greyscale typically cuts a scan by a third to a half with no loss of legibility, as long as the document does not rely on colour for meaning (a coloured official stamp, a highlighted clause, a chart).
Check the portal's requirements first. Some explicitly demand colour scans of ID documents.
Step 4: compress once, from the original
Every compression pass re-encodes already-lossy data. Two 60% passes are visibly worse than one 40% pass at the same final size. If you have already compressed and it is still too big, go back to the original file and start again with harsher settings, rather than compressing the compressed version.
Step 5: rebuild from images, as a last resort
For a stubborn scanned document, export the pages as JPEGs, compress those images individually where you have fine control, and rebuild a PDF from them. You lose any text layer, but for a document that never had one you lose nothing at all, and per-image control usually beats a whole-document slider.
Common limits worth remembering
| Destination | Typical cap |
|---|---|
| Gmail / most email | 25 MB per message |
| Visa and passport portals | 1–2 MB per document |
| University applications | 500 KB – 2 MB |
| Job application portals | 2–5 MB |
| Government tender uploads | Often 1 MB, occasionally 100 KB |
When a form specifies both a size and a dimension requirement, common for photographs, meet the dimension requirement first, then compress.
What not to do
- Do not rename a JPEG to .pdf. It will be rejected, and on some systems it will be silently accepted and then fail downstream.
- Do not password-protect a file to shrink it. Encryption does not reduce size, and many portals reject protected files outright.
- Do not zip it unless the portal asks for a zip. PDFs are already compressed internally, so the saving is marginal, and most upload forms only accept
.pdf. - Do not compress an already-approved document you may need to submit again. Keep the original.
Putting it together
For a scan that is over the limit, split out only the pages the form needs, then run the compressor with Compress images at Balanced, and step down to Smallest only if the file is still too large. For a digital PDF, try Keep text selectable first, since it keeps the text searchable. If a portal wants a single page as a picture instead, PDF to JPG gives you that page as an image you can resize and compress on its own.