Adobe Acrobat Ridiculous Document Protection – How to prevent an image from being copied from a PROTECTED document?
Solution 1:
The entire suite of options within Adobe's PDF apps is pretty much optional to any other PDF-capable app. There's no way to truly lock a file.
Stack overflow has a list of ways to bypass protection -
How to remove security from a PDF file?
Another is 'just find an app that doesn't respect that particular aspect of protection'. You found one, Preview, there are many more.
Solution 2:
You shouldn't rely on that feature for serious protection of your document. It is simply not possible to restrict access to the image by third party software if you still want the user to be able to view it. I.e. it is not really a bug in Preview, it is a limitation in what's actually possible for Adobe to enforce.
You could take a screenshot of the final, composed product and use that in the PDF instead of your original material. That way the image will be blended together with the rest and cannot as easily be separated again.
Solution 3:
I work in Information Security and always say that once someone can put their eyes on some information, it is gone. There is absolutely no way to prevent the data from leaking if there is such a will.
You can screenshot it, you can learn by heart, you can make a photo - whatever.
The only two protections you can expect is confidentiality against the ones that do not have the (wisely chosen) password of an encrypted file, or integrity of a correctly digitally signed document.
Solution 4:
It seems like you're more concerned about protecting the graphic design elements of your PDF, and not so much about protecting any classified information included the PDF, correct?
If that is the case, I recommend flattening all design elements first (without including text), then export to PDF. This will turn convert all graphic elements into a single image so that none of the design elements can be pulled apart. Next, import the flattened PDF into InDesign (or whatever program you're using), add all text elements over it and re-export.
Let me know if this helps.