PDFs grow large for one main reason: images. A scanned page or a photo embedded at print resolution can be several megabytes on its own, while the text on the same page is a few kilobytes. Compressing a PDF is mostly about making the images smaller.
The two approaches
- Clean-up (lossless). Remove metadata and re-save the file with a more compact structure. Text stays selectable and links keep working, but the saving is usually small unless the file was saved wastefully.
- Image compression (lossy). Each page is rendered to an image at a lower resolution and quality and rebuilt as a PDF. The saving can be large, but text is no longer selectable and quality drops as you push harder.
How to hit a size limit
Government, university and job portals often limit uploads to something like 100 KB, 200 KB, 500 KB or 1 MB. A compressor that targets a size works down a ladder: it tries gentle settings first and gets stronger until the file fits. If it cannot reach the limit, you get the smallest result it could make, and you know to try other steps:
- Split the document and upload only the pages that are required.
- If you made the PDF from images, shrink the images first (see the image compressor) and rebuild the PDF.
- Scan in black and white or at 150 DPI instead of colour at 300 DPI or more.
What you give up
Heavy compression makes text softer and small print harder to read, and removes the ability to search or copy text. If the PDF must stay searchable, use the clean-up level or compress the source images instead of the finished PDF.
Doing it privately
Many compressors upload your document to a server. ToolGalaxy runs the whole job in your browser, which matters for contracts, ID scans and anything with personal data. You can confirm it yourself with the steps in our guide on checking that a tool does not upload files.