You can transfer PDF files and attachments to the Apple Books app.

First select the PDF file or email attachment and then tap on the action menu button. (circle with 3 dots button).

Then select 'Share' from the action menu.

In the share window, scroll along the second row of icons until you see the Books icon and tap on it.

If you do not see the Books icon, tap on the "More/..." button to find it.

This will then send the PDF to the Books app for you to read it.