Q: Password promted every time I try to access a book

Hello Taafe92,

Welcome to the Apple Support Communities!

I understand that you are trying to open a PDF in iBooks but are being prompted to enter your Apple ID and password. Once you enter your information, does it say that your Apple ID or password was incorrect, or does the same sign in prompt simply come back?

If the prompt is returning without an error, I would suggest signing out of the iTunes & App Store on your device and then sign back in. Test the PDF after you have signed back in. 

Sign in with a different Apple ID on your iPhone, iPad, or iPod touch - Apple Support

Have a great day,

Joe