How to use the Protect PDF
- Add the PDF.
- Enter the password people will need to open it. Write it down: it cannot be recovered.
- Under More settings, set a separate owner password and untick printing, copying or editing if you want those blocked.
- Press Protect, then open the copy once to confirm the password works.
Encryption strength
Protection uses 256-bit AES, the strongest encryption that the PDF 2.0 standard defines. Every current reader handles it, from Adobe’s free reader and the viewers built into browsers to Preview on a Mac and phone apps; software older than 2008 may refuse it.
A weak password undoes strong encryption, because a protected PDF can be attacked offline. Use a phrase of four or five unrelated words or at least 12 mixed characters, and send it by a different channel than the file, for example by text message.
What the restrictions really do
Blocking print, copy or edit only takes effect with an owner password that differs from the open password, and it depends on the viewer respecting it. Some free programs ignore these flags.
Form fields can always be filled in, so a protected form still works for the person completing it. For confidentiality, rely on the open password; treat restrictions as a deterrent.
Before you send it
Open the protected copy once and type the password, so you know it works before the original is deleted or the email goes out. A typo in the password field cannot be fixed afterwards.
If the file is too large to email, compress it first and protect it last: Compress PDF cannot read a file that needs a password to open.
Questions
How do I remove the password later?
Use Unlock PDF with the password you set here.
Is the content altered?
No. Everything on the pages stays as it was and is merely encrypted, adding no more than a handful of kilobytes.
Does the password leave my computer?
No. Encryption happens in the page, and nothing is sent to a server.
Can I protect several PDFs with the same password?
Yes. Load them all at once; every file gets the same password and permission settings.