What happens when you use an online compressor
Most free PDF compressors work by upload. Your file goes to the company's servers, it is processed there, and you download the result. Their privacy policies usually promise to delete files after an hour or a day. You have to take that on trust, along with their security, their staff access controls, the jurisdiction their servers are in, and whatever third parties (cloud hosting, analytics, advertising) are involved. That is reasonable for a restaurant menu. It is a different matter for a passport, a payslip, a bank statement with your account number, a medical report or a signed contract.
Why it matters for applications
Visa, Centrelink, rental and employment applications collect exactly the documents identity thieves want, and their upload portals have strict size limits. As reported, ImmiAccount allows about 5 MB per attachment and Centrelink under 10 MB per upload (check the current limits). Phone scans easily exceed them, so people search for "compress PDF" and upload their most sensitive documents to the first result. A local tool avoids that trade.
How in-browser compression works
A modern browser can run real PDF software. This site's compressor uses pdf-lib to rewrite PDFs and pdf.js, Mozilla's PDF renderer from Firefox, to draw pages. Both run in a background worker inside your browser tab. The file is read into memory, processed, and handed back as a download. Nothing is sent to a server, so there is nothing to delete afterwards.
- Light mode rebuilds the PDF without unused objects, thumbnails and metadata; text stays selectable.
- Strong mode renders each page to a JPEG at a resolution you choose, which shrinks scans dramatically; the text becomes part of the image.
- Target size tries several resolutions and qualities and picks the best one under your limit.
How to check nothing is uploaded
- Open the compressor, then press F12 (or Cmd+Option+I on a Mac) and choose the Network tab.
- Compress a file and watch the requests. You will see the page's own scripts and pdf.js's font files, which are downloads. You will not see your PDF going anywhere.
- For proof, switch the Network panel to Offline and compress another file: it still works.
Other private options
- macOS Preview: File → Export, Quartz Filter "Reduce File Size". Effective but aggressive; small text can become hard to read.
- Microsoft Word: if you have the source document, export to PDF with "Minimum size".
- Your scanner app: rescanning at 150 dpi in greyscale often produces a small file from the start.
Good habits with sensitive PDFs
- Keep the original; compress a copy.
- Check the compressed file is legible at 100% before you submit it.
- Delete spare copies from your Downloads folder once submitted.
- When sending identity documents to non-government recipients, consider watermarking them with the purpose and date.
On a phone
The compressor runs in a phone's browser too, which matters because phones are where most document photos are taken. Make the PDF on the phone, compress it on the phone, and upload it from the phone, and the document never touches a third-party server along the way.
What to do if a portal still rejects the file
If the compressed PDF is under the limit and the portal still refuses it, check the other rules: some portals reject encrypted PDFs, files with unusual characters in the name, or PDFs over a page count. Remove any password with the Unlock tool (you need the password), rename the file with plain letters and numbers, and try again.
Why not just email it to yourself?
Some people shrink documents by emailing them to themselves, relying on the mail app's "reduce size" option. That only works for images attached to the email, not for PDFs, and it still puts the document on a mail server. Compressing locally is quicker and leaves no copy behind.