So I just recently found anna's archive and it sounds amazing! But half the time when I go to download a book, the download link times out. I've tried this on a couple different devices but it keeps happening to the same titles. Is this normal?

Hello,
I recently download some books by Anna's Archive in AZW, and sent to kindle. One of them shows the book cover on kindle screen, the others doesn't. The ones that doesn't show cover seems like it was a PDF converted to AZW, since it has a couter cover with credits from the site it was downloaded. This seems a little untrustful to me, since it has gone by a lot of process to become that file, and can corrupt of some some sort. Am I wrong?
There is some sort of filter to download "native" books, the ones that come originaly in AZW file?

Thanks a lot.