Write on PDF does not collect, upload, or share your PDFs, annotations, or passwords. Everything runs in your browser on your device. There is no server, no analytics, and no accounts.
Saving a password-protected PDF writes an unlocked copy: the password and restrictions are removed from that file. Printing decrypts the PDF in memory for the print dialog; the extension does not download a file when you print. The password itself is kept only in memory while the document is open, and is never uploaded.
Annotations, saved signatures, and preferences stay in Firefox local storage on your computer. You can erase them by removing the extension.
Full policy: https://codekp.com/apps/write-on-pdf/privacy.html