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  1. Adobe offers a 7-days trial of its Acrobat Pro. It's technically free but not terribly useful in the long run.
  2. LibreOffice can export a file as PDF, but only with 128-bit RC4 encryption. Obsolete.
  3. PDFMate PDF Converter provides only 40-bit encryption.
  4. There are various websites I did not even check because uploading unprotected sensitive data to the Internet is the opposite of protection in my opinion.

So, any further ideas?

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  • Use Apache's pdfbox jar file and do it programmatically with Java?
    – ivanivan
    May 24, 2019 at 14:29
  • Are you still looking for an answer/solution? Did you look at pdfbox as @ivanivan suggested?
    – Ryan
    Jun 17, 2019 at 19:06
  • @Ryan: I found my answer, but not the one ivanivan suggested. Maybe I should answer my own question while I'm on it anyway.
    – sigil
    Jun 18, 2019 at 15:23

2 Answers 2

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Sejda Desktop includes a Protect PDF tool that can encrypt your PDF using AES 256 bits. The file is processed on your computer, not the cloud.

Sejda Desktop is free to use for documents up to 200 pages or 50 Mb, and 3 tasks per day.

The Protect PDF tool is also available as an online service. You mentioned you would rather not upload the document online, but at least you can check the tool's features before downloading the desktop app.

Disclaimer: I'm one of the developers.

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  • Thank you very much for your desire to help. Unfortunately, 3 free attempts per day is hardly sufficient for any nontrivial piece of software. I would spend them just trying to figure out how to use it.
    – sigil
    May 25, 2019 at 15:37
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Through lots of trial and error I found the answer myself: QPDF. It's command line only but sensibly documented and does the job. Acrobat Reader DC confirms that encryption is AES-256.

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  • In the absence of any objections, I'm accepting my own answer.
    – sigil
    Jun 19, 2019 at 13:14

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