Enhancements for book download

Hi! Another PHP developer here 😁 I noticed two things that can be improved on the backend while downloading Make Book:

1. Please don’t name the file doc-1.pdf—my Downloads folder on Mac is already a mess, and I don’t want to make it even messier haha.

2. Right now, the file is streamed (i.e., there is no Content-Length header), which means the browser doesn’t know where the end of the object it’s downloading is. It’s not a problem when you have good internet connection, but if it’s really bad downloading streamed object is annoying (you never know when you can leave this cafe with shitty wi-fi)