Protect PDF encrypts a file with AES-256 and a password needed to open it. You can also choose whether people may print, copy text, modify it or fill forms and comment. Keep the password safe: if you forget it, we can't recover it. Unlock PDF does the opposite for files you have the right to open: it removes owner restrictions on printing, copying and editing, and if the PDF asks for a password to open, you type it once and it is removed. It does not guess or crack passwords.
Sign PDF lets you draw your signature with a finger or mouse, type it in one of three styles, or upload an image of it with the option to remove the white background. Place it on any page, as many times as needed, and add your name or the date. It is a simple visual electronic signature, not a certificate-based digital signature, and it is for signing yourself; it doesn't send requests to other people.
Redact PDF permanently hides information. Draw boxes over what must disappear, and the affected pages are converted to images with black boxes, so the hidden text is truly deleted rather than covered. Those pages lose selectable text (you can run OCR afterwards), and document metadata such as title and author is removed by default.
Flatten PDF turns form fields and annotations into a fixed part of the page, so they look the same but can't be edited or deleted in a PDF reader. It's a good final step before sending a completed form. Compare PDF checks two versions of a document: text mode highlights added words in green and removed words in red page by page, and visual mode overlays the pages and marks changed pixels in red.
A useful habit for sharing documents: redact first, then protect. Redact PDF removes personal data such as ID numbers or account details, and Protect PDF makes sure only people with the password can open the result. For signed forms, sign, flatten and then protect. If someone sends you a new version of a contract, Compare PDF shows exactly what changed before you sign it.
Flattening is not a lock: to stop others from opening a file, use Protect PDF, and to remove information, use Redact PDF.