PDF · 6 min read
How to compress PDFs without losing quality
Most PDFs are bigger than they need to be — usually because someone embedded a 12-megapixel scan when a 1-megapixel version would look identical at reading size. Here's how to get a smaller file that still looks great, without uploading anything to a sketchy website.
By UltraConvert Editorial · Published · Updated
Why PDFs get bloated in the first place
There are usually two culprits: oversized images and uncompressed fonts. A photo embedded at 600 DPI takes 36× more space than the same photo at 100 DPI — but on a screen, you can't tell the difference. Scanners and phone-camera PDF apps love to dump 600+ DPI images into PDFs by default. Fonts are the other problem: a PDF often embeds the entire font file (every character, including ones not used) instead of just the glyphs in the document.
The simple fix: re-compress the images inside
Most PDF compressors work by re-encoding the embedded images at a lower DPI and quality, then optionally subsetting the fonts. The trick is finding settings that drop the file size by 60–90% without making text fuzzy or photos blocky. For documents read on screen, 150 DPI is the sweet spot — visually identical to 300 DPI for most readers, but a quarter of the bytes. For printed documents, stick to 300 DPI.
What to do for different file types
Scanned text documents: convert to 'text PDF' (aka searchable PDF) if possible — that turns the page from an image into actual text, which compresses 50× better. Photos and design work: 200 DPI JPEG quality 80 is the standard. Mixed documents (text + photos): 150 DPI for the document, but bump photos to 200 DPI on the pages where it matters.
When to use a different format
PDF is great for documents that need to look identical everywhere. But if you're sharing photos, JPEG/WebP/AVIF will give you 5-10× smaller files for the same visual quality. If you're sharing text-heavy reports, plain HTML or Markdown are tinier still. Use PDF when the layout matters; otherwise, switch formats.
Doing it without uploading
Online PDF compressors have a privacy problem: they upload your file to their server, do the work there, and send it back. For confidential contracts, financial reports, or medical scans, that's a leak risk. Browser-based compression (the kind we run here) does the work locally — your file never leaves your device. The output is identical; the privacy is not.