How to check if a website uploads your file
Every modern browser can show you each request a page makes. If a tool uploads your file, the upload appears in that list, about the size of the file. The check takes five minutes and needs nothing installed.
Before you start
Use a file you do not mind sharing, such as a PDF of a public leaflet, and make it a few megabytes so it stands out in the list. Note its size.
Step by step
- Open the tool's page and let it finish loading.
- Open the developer tools. In Chrome, Edge and Firefox press F12, or Ctrl+Shift+I (Cmd+Option+I on a Mac). In Safari, first turn on the Develop menu under Settings, Advanced, then press Cmd+Option+I.
- Choose the Network tab. Tick "Preserve log" (Firefox calls it "Persist Logs") so the list survives a page change, then clear the list.
- Run the tool. Choose your file, set the options and click the action button. Watch requests appear.
- Sort by size and look at the method column.
What an upload looks like
An upload is a request with the method POST or PUT, sent to the service's own domain or to a storage provider, whose size is close to the size of your file. Click it and open the Payload or Request tab: you will see the file name or a block of binary data.
Large files are often sent in pieces. In that case you see a run of POST requests of equal size that add up to your file.
What a browser-based tool looks like
You see GET requests for scripts, styles and fonts when the page loads, and then little or nothing while the tool runs. No request comes near the size of your file, and the result appears as a download with no matching request at all, because it was built in memory.
Try it on Merge PDF on this site. While the page loads you will see the pdf-lib and pdf.js libraries arrive from cdnjs, followed by the pdf.js worker that draws the thumbnails. When you click Merge PDFs, the list stays as it is.
A second test: switch the network off
Load the page, then turn off Wi-Fi or tick "Offline" in the Network tab, and run the tool. A tool that works with no connection cannot be uploading anything.
Two cautions. Tools that draw pages fetch one more file the first time they run, and OCR PDF needs its language data, so those must be run once while online before the offline test means anything. Delete Pages needs nothing after the page has loaded, and Merge PDF fetches its last file, the worker that draws the thumbnails, as soon as the page has loaded, which makes them the clearest to test.
What this check does and does not prove
- It shows what happened during this visit, with this file. A site can change its code tomorrow.
- Watch the whole session. An upload could begin after the result appears, so leave the panel open until you close the tab.
- Look at the WS filter as well. A few services send data over a WebSocket, which is listed separately.
- Small requests still carry information. Analytics calls can include the name of the tool you used, though not your file.
For a document that matters, run the check again on the day you use the tool.
Frequently asked questions
Do I need to install anything to check?
No. The developer tools are part of Chrome, Edge, Firefox and Safari. On Safari you turn on the Develop menu once under Settings, Advanced.
Can I do this check on a phone?
Not easily. Mobile browsers do not include the Network panel. Run the check on a computer, or use the offline test: load the page, switch to flight mode, and see whether the tool still works.
What if I see requests to Google or a CDN?
Requests to a content delivery network fetch the libraries a page needs and are normal. Requests to analytics or advertising services report that a page was viewed. Neither contains your file; check the size and the payload to be sure.
Sources
- Inspect network activity (Chrome for Developers): how to watch every request a page makes in the Network panel.
- Network Monitor (Firefox Source Docs): the same check in Firefox's developer tools.
- File API (MDN Web Docs): a web page can read a file only when you choose it in a file picker or drop it on the page.
Tools used in this guide
More guides
- Are online PDF tools safe? What happens to your file
- How to merge PDF files without uploading them
- How to compress a PDF for email
- How to compress a PDF to a specific size
- How to redact a PDF properly
- Can a redacted PDF be unredacted?
- How to make a scanned PDF searchable
- How to password protect a PDF
- How PDF tools work in a browser without uploading anything
- How to fill out a PDF that is not fillable
- PDF to JPG or PNG: which one to choose
- How to remove metadata from a PDF
- Split or extract PDF pages: which tool to use
Or see all guides and how this site compares with other PDF services.