In addition to what everybody else said, there's also trying to find pirated material for niche markets. That is, it's hard to find. I sell an e-book as well, and have foregone any DRM.

More to your point, could people buy the book and give it to their friends for free? Absolutely. And I'm sure some people have. Again, I'm okay with this. I'd rather people bought the book but like print, I can't stop people from sharing and ultimately, I don't want to.

Some people don't care about getting books for free. They just care about the convenience of getting the book.

If you make the book more convenient to get than downloading it for free, the more likely you will make money off of them.

If you find a copy of your book posted on another website where people can download it, what can you do? Presumably the webmaster won't be the nicest guy in the universe to deal with.

Write a bit of code that automatically inserts a footer on every page with details of who bought it.

"this is The personal copy of Tom Wainworth - tom@wainworth.eu"

You can require a password to download the book in the first place, which is like a "please don't steal" sign. Many people won't. Those who will can't be stopped by any technological means; DRM doesn't work, and devalues the product for honest customers.

You can hope that the freeloaders end up increasing the book's popularity and attracting more paying customers. But there isn't much you can do to influence that except make good stuff.

You can't get around this. People will pirate it and they do. However, those who pirate it wouldn't pay money anyway so there's no loss.

And because it's literally two clicks to buy it with PayPal, it's easier to buy it on my site than to try and find a pirated copy, so for those who want to buy it legally it's really easy.