Why here
Most PDF sites upload your file to their server, do the work there, and keep a copy for a while. These tools do the work inside the page instead, using PDF.js (the reader built into Firefox) and pdf-lib, so the file never leaves your device. There is no queue, no daily limit and no watermark.
Questions
Do I need to install anything?
No. Any up-to-date browser on a computer, tablet or phone will do.
What about password-protected PDFs?
A PDF that needs a password to open, or is encrypted against changes, can't be edited here. Remove the protection in the program that made it first.