WARNING in coding I am a Noob and I have been at this for three days.

I have a very simple setup. I literally have some pdfs that I upload to github, then username_.github.io/book/filename_.pdf and it works perfectly! But when I add a custom domain, firstly it moves it to main branch and seems to delete the branch I had it in, and then it won’t open the files, 404 error. I purchase website.download to look more professional and serve the PDFs with my brand in the URL, for delivering the files. Any assistance would be great, perhaps I need to index with html file? But I haven’t got a clue…

Thanks in advance to any assistance offered!

It tooks me three full days to just work out how to get the PDFs loaded and served at URL, to view them before downloading. Then I get to add domain, which I could do easily, only to not have it work with the custom domain.