PDFGeek

PDFGeek merges, splits, rotates and watermarks PDF files on your own machine.
The online tools cap a free merge at a handful of files and charge for anything more.

PDFGeek has no cap. Nothing is uploaded, there is no account, and the documents never leave the computer they are on, which matters for anything under NDA.

No Pro tier, no subscription, no ads and no bundled offers. Windows 10 1809 or later, 64-bit. Free to use, including at work.