Selling an electronic pdf for profit is no different than asking money for one’s emails.
By the way, I charge $8.79 to read this electronic post. Got PayPal?
I am not sure what your point is? Most commercial eBooks intended for Kindles or other eBook readers are not in pdf format, they are generally in epub, mobi, or azw format with some sort of DRM (digital rights management) protection. They are typically more compact than a pdf file and have more viewing options. This is helpful when you start getting older and a larger font is easier for you to read. And people, lots of people pay lots of money to read them. We have friends who have gotten a pretty good amount of royalties from their eBook sales. They generally make more money from their eBook sales than the paperbacks.
Personally, as a prolific junk collector, I have found that I am better off in most situations purchasing electronic books. They are cheaper, do not take up space and if you buy them from Amazon, you can download a book you have purchased to just about any device when you are out and about. A lot of times if you buy a hard copy on Amazon they will sell you the eBook for just a dollar or two extra. If you lend an eBook to someone, they automatically come back to you after a prescribed amount of time. If you check an eBook out from the library you do not have to worry about a fine or going back to the library. They have a lot of advantages for me.