Online PDF tools have to upload your file to their servers before they can merge, compress or convert it. Reputable services encrypt the transfer and delete files after a few hours, but you are still trusting a third party with the document. For contracts, IDs, medical or financial records, an app that processes the file on your phone, such as ProperPDF, avoids the upload entirely.
What an upload means
When you drop a PDF into a website, a copy travels to a server, gets processed there and is sent back. The service's privacy policy decides how long that copy is kept, who can access it and where it is stored. Most large services say they delete files within hours. That is a promise, not something you can check.
When it's worth avoiding
A flyer or a public brochure doesn't need protecting. A passport scan, a payslip, a tax return, a lease or a medical letter does. If a leak of the file would worry you, do the work on your own device.
How to keep the work on your phone
Use an app that states it processes files locally, and check it still works in airplane mode. ProperPDF merges, splits, compresses, signs and locks PDFs that way on iPhone, iPad and Android. Share only the pages the recipient needs, keep the original, and if you password-protect a file, send the password through a different channel than the PDF.
Questions
How can I tell if a PDF app uploads my files?
Turn on airplane mode and try the tool. If it still works, the processing happens on your device.
Does ProperPDF upload my documents?
No. ProperPDF processes PDFs on the phone. The website doesn't accept uploads either.