I can’t download or read PDFs on my new iPhone 14 Pro

What email app are you using?

If using Apple's default Mail App,

tapping the PDF itself or the icon if it's a multi-page PDF will open the PDF in a short of preview, but will not download it anywhere.

You can use the share icon, to move it to a different App, or save it into the Files app.

If the download within the email message is not completing, then check your internet connection. Try force closing the app and opening it again, and see if it decides to download the PDF.

Download from where?

In what specific App?

Have you checked the Downloads Folder in the Files App?

Please provide some details, and someone may be able to help.