Cannot download my pdf files from email

Hi diliaram,

For help with saving a PDF, test the steps below found in the article here: Save and edit PDFs to the Books app on your iPhone or iPad - Apple Support

"Save a PDF attachment on your iPhone or iPad

1. Tap the PDF to open it.
2. Tap the Share button .
3. Swipe left over the app icons and tap Books.
4. If you don't see Books, tap the More button. Then tap Books.
5. 

Your PDF opens and automatically saves in the Books app. You can find it later in the Library tab.

To access your files on any of your other devices, make sure that you set up iCloud Drive."

You can also find helpful information here: Read PDF documents in Books on iPhone - Apple Support

Cheers.