Copy protection is difficult, aside from high level encryption.
The best way to approach this, if trying to protect the work (and potentially make a profit) is to follow the path of companies that sell plans for home-built aircraft.
They know that copies will be made. They know that people will try to resell their $300 set of plans. They know that they're going to lose money.
So, they do two things:
1. Anything publicly release has intentional 'mistakes' in the prints/plans (such as "sample" drawings shown on websites or in books). Some are big, like changing all of the dimensions for the left wing, so that anyone building from the 'public' copy is going to build one wing two feet too short and with three ribs missing. Some 'mistakes' are small, but numerous: Like specifying the length of every stiffener some random amount too short or too long - but within 1/2", or so.
2. EVERY copy of the plans gets a unique serial number, plus hidden identifying data. This way, if a copy of the plans shows up online somewhere, they can track down the original purchaser and pursue legal action. You cannot, of course, reveal what the data is, or how it's being hidden.
Just one example of many ways used to hide identifying data in aircraft plans, is using simple "revision numbers" or characters claimed to be for 'internal use only' in something like a drawing reference key on each page:
Code:
Spar cross section: Dwg-345-678-A(IV)
Rib forming buck: Dwg-345-679-AA(III)
Rib doubler: Dwg-345-983-CG(IX)
...Etc...
...Where the Roman Numerals represent the serial number, or 'hidden'/second serial number.
There's a company in Canada that goes so far as to assign internal serial numbers, along with the visible serial numbers. Somehow, they include the hexadecimal check sum of the drawing number, visible serial number, and internal (hidden) serial number in each drawing. It makes every page unique, identifiable, and traceable; even though identifying the check sum is not easy, since it's different on every page.
And it means that any page without a correct check sum has been adulterated or copied and modified (both being copyright issues).