Are online PDF tools safe? Upload-based versus in-browser tools

Free PDF websites are convenient, and most people use them without asking where their document goes. For a restaurant menu that does not matter. For a contract, a bank statement or a passport scan, it matters a great deal.

Two very different designs

Upload-based tools send your file to a server. The server processes it and sends a result back. This design lets a site use heavy software that would not run in a browser, but it means a copy of your document exists on someone else's machine, at least for a while.

In-browser tools do the work on your own device. The page downloads the program, and the program reads your file locally. The document is never sent anywhere, because there is no server in the middle that needs it.

Both designs can be run responsibly or irresponsibly. The difference is what you have to trust. With an upload-based tool you are trusting the operator's retention policy, security and honesty. With an in-browser tool you are trusting only that the code does what it says, and you can check that.

How to tell which kind you are using

• Watch the network.** Open your browser's developer tools, go to the Network tab, and process a small test file. If a large request leaves your computer carrying the file, it is being uploaded.
• Try it offline.** Load the page, switch your connection off, and run the tool. A tool that works without a connection is processing locally.
• Read the policy.** A site that uploads files should say how long it keeps them and who can access them. Vague wording is a warning sign.

Questions to ask of any upload-based service

1. How long are files kept, and is deletion automatic?
2. Is the connection encrypted, and are download links guessable?
3. Is the document ever read by a person, or used to train anything?
4. Which company operates the service, and where?

When to avoid online tools altogether

Even a well-run service is not the right place for material you could not send by email: medical records, documents under legal privilege, unreleased financial results, or anything covered by a confidentiality agreement. Use desktop software that never touches the network for those.

Practical habits that help with any tool

• Remove metadata and hidden pages you do not intend to share.
• Use a copy, never your only original.
• Be wary of sites that require an account or an email address before they will process a file, since that links your documents to your identity.
• Check results before you send them on.
• Keep your browser updated, because the browser is doing the security work for in-browser tools.

The limits of in-browser tools

Running locally has costs. Very large files are limited by your device's memory, and some jobs, such as faithful conversion of complex Office documents, are better done by desktop applications. A good in-browser tool tells you plainly where its limits are, and you should treat conversions of important documents as drafts until you have checked them.