Yeah, sadly, there's no good way to protect a PDF from folks who want to access the document. Like a house key, password protection only keeps out the "mostly honest" folks.
Remember, if you did rasterize the PDF, and make it a PDF of an image, all the person has to do is to either print that, and then scan it, then OCR it. Alternatively, many of the screenshot applications out there can OCR directly within a screenshot.
And yes, you nailed it; this is "just part of creating a digital product." Perhaps if you only sell it to honest people? :>)
Actually, I just thought of something. I wonder if it's possible for a "key" to be emailed to the person opening the file — if it's been registered for that person. Without the key, the file cannot be opened. Using the Key, the file would open to the first page with an option to enter your email and send that off. When received by the company who created the key, a return number would be sent that has a one-time use for opening the file. As you point out, this would not stop GoodNotes from generating a new copy, but if I paid $$, I'd be a lot less likely to generate a clean copy.
I am unaware of anthing like this out there, or even if it's possible. Maybe someone who comes across this can make it happen. In the meantime, just cross your fingers like everyone else; put a password on it and cross your fingers. Sorry
This "key" just seems like an unnecessary additional step. How is that any different from paying and getting the link to the file directly?
If you really want to limit who can open a file you need to use DRM technology, which is very expensive and not worth it for most people. This type of technology allows a file to be opened only on specific devices, and prevents most forms of copying and re-creating it.