Isn't distributing your ebook as a simple pdf an invitation for people who don't want to pay for it to copy and circulate it for free? How do you get around this?
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.
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.
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.
I don't have a general download link, when you buy it I'll send out an email with a personal download link that will expire in a week or so, and can only be used 5 times.
Of course you can just write to my support email box and I'll send you a new link if you loose the file. I also send out free updates whenever I update the book.
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.
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.