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Protect PDF

Add a password to a PDF entirely in your browser. No upload, no account, no server — the file and the password never leave your device.

Frequently asked questions

How does Protect PDF work?
The PDF is read in your browser, encrypted with the password you type, and written back out as a new file that you download. The encryption happens in JavaScript on your own device. Neither the file nor the password is sent anywhere, which matters more here than on most tools: uploading a document to add a password means handing both the document and its password to a stranger's server.
Is the password recoverable if I forget it?
No, and that is the point. The password is used to derive the encryption key; it is not stored in the file in any readable form. If you lose it the document cannot be opened by us or by anyone else. Write it down somewhere before you close the tab. This is why the tool asks you to type it twice.
What do the print and copy checkboxes actually do?
They set permission flags inside the PDF. Well-behaved readers such as Acrobat and Chrome honour them and will grey out printing or text selection. They are enforced by the reader, not by cryptography, so a determined user with the right software can bypass them. The password is the part that genuinely protects the file; treat the permission flags as a signal of intent rather than a lock. Printing and copying are the only two things restricted: a protected form stays fillable, annotations still work, and assistive technology can still read the text.
Can I add a password to a PDF that already has one?
Not directly. An encrypted PDF cannot be read without its existing password, so there is nothing to re-encrypt. Remove the current password at PDF Mavericks' Unlock PDF tool first, then add the new one here. The tool tells you this rather than failing with a parsing error.
What encryption does it use?
128-bit AES, the standard security handler defined by the PDF specification, applied through the same library the Unlock PDF tool uses to read protected files. AES is only available to files that declare PDF 1.6 or later, so an older file (many scanners and government portals still produce PDF 1.3) has its header raised before encryption rather than falling back to the 40-bit cipher those versions imply. The output is an ordinary encrypted PDF: Acrobat, Preview, Chrome and every other standards-compliant reader will prompt for the password and then open it normally.
Which characters can the password contain?
Letters, digits, punctuation and accented Latin letters: the Latin-1 range. This is a limit of the PDF format itself, not of this tool: the standard security handler stores the password as single-byte characters, so Devanagari, Chinese, Cyrillic and emoji cannot go into a PDF password at any tool. The tool checks before it encrypts and tells you, rather than producing a file you could never open again. Only the first 32 characters count, which is also a rule of the format.
Does it change the contents of my document?
No. Pages, page sizes, rotation, text, images, form fields and their values, annotations, and the document's own title, author and producer are all preserved. What the tool adds is the encryption dictionary; the only other change is that a file declaring a PDF version older than 1.6 has its header raised to 1.7, which is what allows AES to be used at all. The page count shown after processing is the count from the file you supplied, so you can check it matches.
Is there a file size limit?
There is no limit imposed by the tool. The practical ceiling is your device's memory, since the whole document is held in the browser tab while it is encrypted. Ordinary documents of a few hundred pages are not a problem on a normal laptop or phone.
Does this work offline?
Once the page has loaded, yes. The encryption runs locally, so you can disconnect from the network before adding the password and the tool still works. That is a reasonable way to satisfy yourself that nothing is being transmitted.

Related Tools

Unlock PDF

The reverse — remove a password you already know.

Redact PDF

Cover sensitive content before sharing a document.

Remove PDF Metadata

Strip author, software and timestamps from a file.

Frequently asked questions

How does Protect PDF work?
The PDF is read in your browser, encrypted with the password you type, and written back out as a new file that you download. The encryption happens in JavaScript on your own device. Neither the file nor the password is sent anywhere, which matters more here than on most tools: uploading a document to add a password means handing both the document and its password to a stranger's server.
Is the password recoverable if I forget it?
No, and that is the point. The password is used to derive the encryption key; it is not stored in the file in any readable form. If you lose it the document cannot be opened by us or by anyone else. Write it down somewhere before you close the tab. This is why the tool asks you to type it twice.
What do the print and copy checkboxes actually do?
They set permission flags inside the PDF. Well-behaved readers such as Acrobat and Chrome honour them and will grey out printing or text selection. They are enforced by the reader, not by cryptography, so a determined user with the right software can bypass them. The password is the part that genuinely protects the file; treat the permission flags as a signal of intent rather than a lock. Printing and copying are the only two things restricted: a protected form stays fillable, annotations still work, and assistive technology can still read the text.
Can I add a password to a PDF that already has one?
Not directly. An encrypted PDF cannot be read without its existing password, so there is nothing to re-encrypt. Remove the current password at PDF Mavericks' Unlock PDF tool first, then add the new one here. The tool tells you this rather than failing with a parsing error.
What encryption does it use?
128-bit AES, the standard security handler defined by the PDF specification, applied through the same library the Unlock PDF tool uses to read protected files. AES is only available to files that declare PDF 1.6 or later, so an older file (many scanners and government portals still produce PDF 1.3) has its header raised before encryption rather than falling back to the 40-bit cipher those versions imply. The output is an ordinary encrypted PDF: Acrobat, Preview, Chrome and every other standards-compliant reader will prompt for the password and then open it normally.
Which characters can the password contain?
Letters, digits, punctuation and accented Latin letters: the Latin-1 range. This is a limit of the PDF format itself, not of this tool: the standard security handler stores the password as single-byte characters, so Devanagari, Chinese, Cyrillic and emoji cannot go into a PDF password at any tool. The tool checks before it encrypts and tells you, rather than producing a file you could never open again. Only the first 32 characters count, which is also a rule of the format.
Does it change the contents of my document?
No. Pages, page sizes, rotation, text, images, form fields and their values, annotations, and the document's own title, author and producer are all preserved. What the tool adds is the encryption dictionary; the only other change is that a file declaring a PDF version older than 1.6 has its header raised to 1.7, which is what allows AES to be used at all. The page count shown after processing is the count from the file you supplied, so you can check it matches.
Is there a file size limit?
There is no limit imposed by the tool. The practical ceiling is your device's memory, since the whole document is held in the browser tab while it is encrypted. Ordinary documents of a few hundred pages are not a problem on a normal laptop or phone.
Does this work offline?
Once the page has loaded, yes. The encryption runs locally, so you can disconnect from the network before adding the password and the tool still works. That is a reasonable way to satisfy yourself that nothing is being transmitted.