Hi! Sorry for the trouble. Regarding the download, that's either a complication with Itch.io (there's an insane number of downloads from the site going on right now, so that's a possibility) or something with the download manager on your computer.
Regarding opening the PDF, try opening it with a Firefox extension or even just dragging-and-dropping it into a Firefox (or Chrome) tab. One of these should work.
Let me know if you continue to have any trouble! Thanks for purchasing.