Compressing a PDF or resizing a photo often means handing over documents full of personal details: ID copies, payslips, bank statements and signatures. It is worth asking where the file goes before you click.
Two ways online tools work
Upload-based tools send your file to a company’s server, process it there and send back the result. Your document exists on someone else’s computer, even if only for a short time.
On-device tools run in your browser. The file is processed by your own phone or computer and does not leave it. The CometBits tools work this way, so your file is not uploaded.
How to tell the difference
- Read the tool page. A trustworthy site says clearly whether files are uploaded.
- Look for phrases about files being deleted after a set time. That usually means upload.
- Test it. Open the tool, then turn off your internet connection and process a file. If it still works, the processing is on your device. Some tools need to download a program or language data first, so load the page fully before you test.
- Check the network activity in your browser developer tools if you are comfortable doing so.
Questions to ask about any site
- Does it say who runs it and how to contact them?
- Does it explain what happens to your files?
- Does it ask for an account just to compress a file?
- Is the page served over HTTPS, shown by a padlock in the address bar?
- Does it push you to install software or browser extensions?
Reduce what you share
Even with a trusted tool, take sensible steps.
- Remove pages you do not need with Split PDF.
- Black out details the reader does not need using Redact PDF.
- Strip location data from photos with the Remove EXIF tool.
- Add a note on ID copies saying what they are for. See our guide on adding watermarks to photos and PDFs.
- Lock sensitive PDFs with a password using Protect PDF.
After you finish
- Delete downloaded copies you no longer need.
- Empty the recycle bin or recently deleted folder on shared computers.
- Do not process sensitive files on a public or shared computer, and clear the downloads folder if you must.
What no tool can promise
No website can guarantee absolute safety, and nobody can promise a perfect result. Your device also needs to be secure, with an up-to-date browser and no unknown extensions that read pages.
A simple rule
If a document would harm you if it leaked, prefer on-device tools, share the minimum, and use passwords and redaction. When you can, use the PDF compressor on a copy, and keep the original offline.