Is there a tool that can add a password to a PDF file?

Question

I've found lots of info on decrypting PDF files, but none on encrypting them. What tool can do this for me?

Edit: There are two good answers below, but in my specific case the PDF was a scan, not a document that I'd created.

Answer 1

Use pdftk. From its man page:

Encrypt a PDF using 128-Bit Strength (the Default) and Withhold All Permissions (the Default)

$ pdftk [mydoc].pdf output [mydoc.128].pdf owner_pw [foopass]

Same as Above, Except a Password is Required to Open the PDF

$ pdftk [mydoc].pdf output [mydoc.128].pdf owner_pw [foo] user_pw [baz]

Same as Above, Except Printing is Allowed (after the PDF is Open)

$ pdftk [mydoc].pdf output [mydoc.128].pdf owner_pw [foo] user_pw [baz] allow printing

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pdftk has a lot of other uses as well and is worth installing if one works extensively with PDF files. To quote from the write-up in Synaptic:

If PDF is electronic paper, then pdftk is an electronic stapler-remover, hole-punch, binder, secret-decoder-ring, and X-Ray-glasses. Pdftk is a simple tool for doing everyday things with PDF documents. Keep one in the top drawer of your desktop and use it to:

• Merge PDF documents
• Split PDF pages into a new document
• Decrypt input as necessary (password required)
• Encrypt output as desired
• Fill PDF Forms with FDF Data and/or Flatten Forms
• Apply a Background Watermark
• Report PDF on metrics, including metadata and bookmarks
• Update PDF Metadata
• Attach Files to PDF Pages or the PDF Document
• Unpack PDF Attachments
• Burst a PDF document into single pages
• Uncompress and re-compress page streams
• Repair corrupted PDF (where possible)

Answer 2

If you use LibreOffice and export a document as PDF (Menu > File > Export as PDF), there's the option to set a password:

Answer 3

qpdf can encrypt and decrypt pdfs. And linearize them, too.

Answer 4

One possible solution is to use jpdf Tweak.