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How Can You Be Sure Your File Never Leaves Your Device?

You do not have to take our word for it

Almost every online converter says "your file is safe". Instead, we want to give you ways to check for yourself. The tests below are ordered from the strongest evidence to the weakest. None needs technical knowledge, and you can do the first two in a few minutes.

1. The airplane-mode test

For a file to be sent to a server, an internet connection is needed. If the conversion works with the internet switched off, that is the clearest evidence that the file is not being sent anywhere:

  • Open the page of the tool you want to use on udfpdf.com and wait until it has fully loaded.
  • Switch on airplane mode on your phone, or turn off Wi-Fi or unplug the network cable on a computer.
  • Choose your file and convert it. The process completes and you can download the result.
  • If you like, reload the page while offline: the site opens from the cache and keeps working.

One exception: modules downloaded on first use

Not every part of every tool is downloaded when a page opens; this keeps the site fast. Parts such as the PDF reading module, the readers that convert Word/ODT/RTF files to UDF, the modules that decode TIFF and iPhone photos (HEIC), and the text recognition engine are downloaded once, the first time you use the related tool. So if you are converting a PDF to UDF for the first time, run the test once with the internet on, then repeat it in airplane mode.

These downloads are only the site's own program files: they are the same for everyone and carry no information about your document. Once downloaded, they are kept in your browser and also work offline.

2. Your browser's Network tab

On a computer, the Network tab in your browser's developer tools lists every request the page makes. If your file were sent somewhere, it would appear here:

  • Chrome and Edge: press F12 (⌥⌘I on a Mac) and switch to the Network tab.
  • Firefox: press Ctrl+Shift+E (⌥⌘E on a Mac) to open the Network tab directly.
  • Safari: first enable developer features in Safari Settings, Advanced, then choose Show Web Inspector from the Develop menu and switch to the Network tab.
  • Clear the list (the cross or bin icon), then choose your file and convert it.
  • You will see only program and font files coming from udfpdf.com (often nothing at all). Check that there is no request that carries the name or size of your file, goes to another address, or sends data (POST).

3. What is the security policy (CSP)?

Content-Security-Policy (CSP) is a security rule with which a website tells the browser "this page may only talk to these addresses". The site sets the rule, but the browser enforces it: if code on the page tries to step outside the rule, the browser blocks the request. The rule of udfpdf.com lets the page connect only to its own address (connect-src 'self') and limits form submission to its own address too (form-action 'self'); the site has no form that sends files. So even if code on the page wanted to send your file to another company's server, an ad network or an analytics service, your browser would not allow it.

Because this rule does allow connections to our own address, in the interest of honesty we add this: the udfpdf.com server only serves static files (pages, program files, fonts) and runs no program that accepts or processes files. The proof that nothing is sent to our own server is the two tests above. To see the CSP header, click the first request (the document) in the Network tab and look at the content-security-policy line under the response headers.

4. What does the service worker do?

A service worker is a small browser program that lets the site work offline. Its job is to save the site's own files in your browser's cache and to serve them from there when there is no internet. It does not save, read or send your documents. In Chrome and Edge you can see the service worker and the list of cached files in the Application tab of the developer tools, and delete them all if you want. Clearing the cache does not break the site; the files are downloaded again on the next visit.

When does the site connect to the network?

Here is the full list:

  • When you first open a page and when the site is updated: pages, program files and fonts are downloaded from udfpdf.com.
  • When you use a tool for the first time: the modules specific to that tool (PDF reading, Word to UDF conversion, TIFF and HEIC decoding, the text recognition engine and language data) and, if needed, extra fonts are downloaded.
  • When you click a link: you go to the site that the link points to (an official UYAP page, for example).
  • When you press Share: the file is handed to the app you choose, and from then on that app's own terms apply.
  • Apart from these, no request is made: there is no analytics, advertising, error reporting or "ping".

The easiest test on a phone

Developer tools are hard to reach on a phone, so the airplane-mode test is the most practical. Open the page, wait a few seconds, switch on airplane mode and convert your file. If it finishes, your file has not left your phone. The only data the site's host keeps is the technical access logs that every website has; we use no accounts, cookies, advertising, analytics or tracking code.