Is there text under those black boxes?
A black rectangle drawn over a name hides it from your eyes and not from a computer. Drop a PDF in and find out which kind yours is.
Drag and drop a PDF here
It stays on your device. Nothing is uploaded — there is nowhere to upload it to.
Open your browser's network panel first, then drop the file in and watch. Nothing is sent. There is no upload endpoint on this site to send it to.