A form rejects your file for being too large, or an email bounces. The fix is usually quick, but the right one depends on what is inside the PDF. This guide explains why PDFs grow so big, which fixes to try first, and when compression does more harm than good.
First, know your limit
Gmail and Outlook.com both cap a whole email, including every attachment, at 25 MB. Job sites, university applications and government portals are much stricter, often 1 or 2 MB per file and sometimes a few hundred kilobytes. Check the exact limit before you start, and check the current size of your file:
- Windows: right-click the file, choose Properties, and read Size.
- Mac: select the file and press Command and I to open Get Info.
- iPhone and Android: in the Files app, press and hold the file and open its info or details.
Why PDFs get so big
A PDF made from a word processor stores text as text, which is tiny: a 20-page report can be under 1 MB. Scanned documents are different. Each page is a photograph, and scanners and phone scanning apps often capture at 300 to 600 dots per inch in full colour, so a single page can take several megabytes. Large embedded photos, such as in brochures and portfolios, have the same effect. Almost every oversized PDF is oversized because of images.
The order to try things in
- Remove what is not needed. Blank backs of scanned pages, cover sheets and appendices the portal did not ask for. Use Split PDF in Extract pages mode and leave them out. Nothing loses quality.
- Split the document if the portal accepts more than one file. Three files of 1.8 MB are better than one heavily compressed file.
- Rescan smarter if you scanned it yourself. For ordinary typed documents, 150 to 200 dots per inch in greyscale is readable and a fraction of the size of a 600 DPI colour scan. Keep colour and higher resolution for photos, stamps or small print, or when the portal requires them.
- Compress what remains. With the PDF compressor, choose Target file size and the limit, and it finds the best quality that fits.
How much resolution you need
Compressing a scanned PDF means lowering the resolution of its page images and saving them as JPEG. The levels in our compressor translate roughly as follows:
- About 144 DPI: comfortable on screen and acceptable when printed. Small print stays legible.
- About 108 DPI: fine for reading normal-sized text on screen. A good default for applications.
- 72 DPI: readable for large text, but footnotes and fine print start to blur.
If the only way to reach a limit is to drop below 72 DPI, the document is probably too long for that portal. Split it, or ask the recipient for another way to send it.
Two kinds of PDF not to compress
- Text-based PDFs that are already small. Compressing by rendering pages as images turns crisp, searchable text into pictures of text, and the file can get bigger. To shrink a document you created yourself, export it again: in Word, choose Save As, PDF and Minimum size; on a Mac, open it in Preview, choose File, Export and the Reduce File Size filter. Both keep the text as text.
- Digitally signed PDFs. A certificate-based digital signature, as used by many e-signature services, is tied to the exact content of the file. Any compression invalidates it. Send the original, or ask the sender for a smaller signed copy.
Compression also flattens fillable form fields and removes the hidden text layer added by OCR, so the result cannot be searched or copied from. Keep the original if you might need either.
Check before you submit
Open the compressed file and zoom in on the smallest text, signatures, dates and any reference numbers. If anything is hard to read, go one step up in quality or split the file instead. A reviewer who cannot read your document will not ask for a better copy.
A note on privacy
Many online PDF tools upload your document to their servers to process it. For bank statements, payslips, passports and medical records, that is a real risk. The tools on this site process the file inside your browser, so the document never leaves your computer or phone.