Can't transfer file to first gen iPad

Maybe I'm an idiot and missing something really simple, but I've been at this for hours now. I recently dusted off my old iPad. I have a book in the form of a PDF I want to read. I cannot for the life of me get a 15mb file into my iPad.

I first tried using Dropbox. But my iPad, upgraded to the latest version of iOS available to it, does not support the Dropbox app. Fine, I'll use Google Drive. No, same problem with the app, and Safari isn't able to open the webpage. Fine, I'll send myself a direct download link. No, I can't download files straight out of email. Fine, I'll download iTunes, the worst piece of software ever invented, to my PC, add the PDF to my books library, and sync it with my iPad. Nope, nothing happens. I don't know why, but nothing.

This level of obsolescence on an expensive piece of technology only five years old is astonishing. Can anyone help me come up with a way to do this?