The simple answer to the above question is: you can't.

Unfortunately there is no way to stop people copying PDFs. Even with 'copy protection', people can print out and scan back in and use as they wish.

I've been selling and giving away digital files online for 10 years now and have found teachers to be a very trustworthy bunch of people and haven't had any issues with this in the past.

You can make clear the licensing arrangements under which you're offering your books and, if you were to find out someone was using them against the license, you could hold them to account.

However, as I said, I've actually never had to do this in 10 years of online business.

The licensing agreement can be clear to the teacher when they buy and you can also print the agreement at the bottom of each page in your original PDF.

These policies can be added when adding your product and will be shown under the product listing before people buy.

Another thing you can do is set a limit on the number of downloads of the original file. You can do this when adding your product. You can also specify a time limit in which they can download the file (eg. 2 days):

Unfortunately, just like for CDs, DVDs, etc. - even online videos can be downloaded and copied - the whole music industry realised that there really is no concrete way of protecting files online.

However, the upside is that the potential for global sales and easy repeat business has far outweighed the concerns now.