Why merging a contract online is a bad idea
The documents people merge are agreements, invoices, scanned identity papers, medical letters and bank statements. Every popular PDF site uploads that file to a server, processes it there, and promises to delete it in an hour.
Even taken at face value, that promise is not the point. The file has already been transmitted, has already sat on a disk you do not control, and has already passed through whatever logging and backup that company runs. For most documents it does not matter. For the ones people actually bring to a merge tool, it can.
This runs in your browser. There is no upload, no queue and no retention policy, because there is nothing to retain. Load the page, disconnect from the internet, and it still works.