I thought, hey, maybe we made an error somewhere, so I went through the process just like anyone else would.
After purchase, I arrived at this window:
Clicked download, and everything went as expected.
Opened the epub file and it opened my ereader. I had the option of selecting what app I wished to use to read the file.
Worked as expected.
I do not know how you would have wound up with a .pdf file and not an epub file. Can you double check that?
What ereader are you using to try to read the book, Jason?