How to Compress a PDF Without Making It Unreadable
Three ways to shrink a PDF that is too big to email, and how to pick the right one so the text stays sharp.
· 5 min read
If you have ever tried to email a 20 MB PDF, you know the "attachment too large" message. The good news: most PDFs can be cut to a quarter of their size without losing anything you care about. The bad news: the wrong method blurs the text or makes it impossible to select.
1. Re-encode the images (the biggest win)
Almost all of a PDF's weight is images. A scanned document or a photo album usually contains huge JPEGs saved at 300 DPI per page. For a document that will be read on screen, 120 to 150 DPI is plenty.
This method has a cost, and you should choose it knowingly: the pages are turned into images, so the text can no longer be selected or searched. That is fine for invoices, scans and brochures. Think twice for a contract you will need to copy text from.
The Compress PDF tool does exactly this. You choose the target DPI and JPEG quality, and you see the old and new size before you download anything.
2. Throw away what is not needed
Some PDFs are bloated because they carry things nobody uses:
- Every glyph of an embedded font, even the ones never printed
- Pages and layers that were deleted but left in the file
- High-resolution preview thumbnails
- Leftover objects from earlier versions of the file
Cleaning these out never hurts quality. It usually saves 10 to 25 percent, less on image-heavy files.
3. Pick the right resolution for the job
The most common mistake is "choose the lowest setting so it gets as small as possible". The target matters:
- Read on screen or send by email: 100 to 120 DPI
- Normal office printing: 150 DPI
- Print shop quality: 200 to 300 DPI
At 72 DPI, 12 pt text is still readable on screen but looks jagged on paper. One step up costs a few hundred KB and is worth it.
When compression does not help
Compressing a PDF that is already optimised, or only has a few pages, can make it bigger. Re-encoding is not a guaranteed win. In that case, keep the original. A good tool tells you when this happens, and ours says so plainly when the result grew.
Your file is processed in your browser the whole time, so there is no upload wait even for large scans.
Open: Compress PDF, free, and your file never leaves your device.