Disclosure: I build UMyPDF, a browser-based PDF toolkit, which comes up at the end of this post. The explanation applies to any tool built either way, and the check in the second-to-last section works on mine as well as anyone else's.
You have a PDF that needs to be smaller. You search for a way to do it, click the first result, drag the file onto a friendly dashed rectangle, and a few seconds later you download a smaller PDF. The whole thing takes under a minute and it works.
The part nobody mentions is what happened in between. On most free PDF sites, that file left your computer, travelled across the internet, landed on a machine belonging to a company you had never heard of ninety seconds earlier, and was written to a disk there. The compression happened on that disk. Then the result travelled back.
That step is not a scandal. It is just how those tools are built, and for a lot of documents it genuinely does not matter. But it is invisible, it is the default, and most people would make a different choice about some of their documents if they knew it was happening. So here is what it actually involves.
The upload nobody mentions
