How to encrypt a scanned PDF book to prevent copying the whole page as image

Question

I have created a scanned PDF and I wanted to watermark and encrypt it to prevent copying and printing.

So I used Adobe Acrobat to apply the watermark and secured the document with 128Bit - AES with password required to print or copy. There are no issues here and PDF File is created and it is a secured document.

However when I open the document in Sumatra PDF it allows option to copy the whole page as image. Like this:

And you can simply paste the image in MS Paint or FastStone Image Viewer. And when you paste both watermark and encryption both disappears defeating the whole purpose of encrypting the document in first place.

See here the result of pasting that copied image:

And here is the PDF Security setting level of the document.

So can some help me how to fix this stupid and frustrating issue?

Answer 1

Unfortunately, "encryption" doesn't work like that. After all, the PDF reader itself must be able to decrypt the image in order to show it on screen, so your document literally carries its own decryption keys. Basically, the only purpose of encryption here is to act as a signal to the reader software that it needs to activate feature restrictions for this document.

Feature restrictions are essentially a "honor system" – they only work if the PDF reader software correctly enforces them. (The document just has a "please don't print me" indicator, but it does not actually "know" whether it's currently being shown or printed.) So if someone happens to have a PDF reader which ignores the restriction flags, nothing you can do about that.

The only real solution here is to apply watermarks to the scanned image itself, so that it will be embedded into the PDF document as a single unit. (For example, you could use ImageMagick to preprocess all scanned pages in batch, and then generate a PDF with all of them. You wouldn't even need Acrobat anymore, just img2pdf.)