What the password really does
The tool uses the standard security handler of the PDF format with AES-256, the strongest method the PDF 2.0 standard defines. A random 256-bit key encrypts the text, images, fonts, form fields and document details of every page, and that key is locked with your password. Without the password the file is scrambled data: no reader can show the pages, search the text or print anything. With it, any reader that supports PDF 2.0 encryption asks for the password once and then shows the document as normal. The pages themselves are not changed, so text stays selectable and the file prints the same as before.
Choosing a password that holds
A protected PDF can be copied and attacked offline, where nobody limits how many guesses are tried, so the password is the whole defence. A name, a birthday or a word with a number at the end is found quickly by guessing tools. Twelve or more mixed characters, or four or more random words, put the file out of reach of any practical attack. The strength meter on this page uses the same checks as our password strength checker, including a list of the 10,000 most common passwords. Send the password by a different route from the file: if the PDF goes by email, give the password by phone or text message.
Print and copy restrictions, honestly
You can also ask readers not to print, copy text, change pages, fill in forms or add comments. These limits are stored in the file and respected by the well-known PDF readers, which grey out the matching buttons. They are not encryption, though: some programs ignore them, and a person who can open the file can always photograph the screen. Use them to signal how a document should be handled, not to keep a secret. The optional permissions password lets you lift the limits later; it must differ from the open password. If you set limits but leave it empty, a random one is used and nobody can lift them, so keep your unprotected original.
Which apps open the protected file
AES-256 encryption has been part of PDF since 2008 and is supported by current PDF readers on Windows, Mac, Android and iPhone, and by the PDF viewers built into Chrome, Edge, Safari and Firefox. Very old readers that only know the earlier 40-bit or 128-bit methods may refuse the file, so ask the recipient to update their reader if it does not open. English letters, digits and symbols work everywhere. Arabic passwords follow the PDF 2.0 rules and open in up-to-date readers, but if the recipient uses an old app, an English password is the safer choice.
Privacy, limits and what to do first
Your PDF and your password stay in this browser tab. The file is read with pdf-lib and encrypted with the Web Crypto API that is built into your browser, and no part of it is sent to NeedMyTools or anyone else. There is no account, no daily limit and no watermark. Files up to 100 MB work on most computers; phones with little memory may struggle with large scans. A PDF that already has a password must be unlocked first, with Unlock PDF and its current password. Protecting rewrites the file, so an existing digital signature will no longer show as valid. Do any merging, splitting, signing or watermarking before you add the password, because a protected file has to be opened again before other tools can change it.