Hi RedGasMan,
Wondering if you are using the "Mail" app when you try to open pdf attachments.
or whether you have a setting in place linking e-mails to another app.
(ie: is Mail app or another app the default for .pdf's)
or
If you have installed a pdf viewer it may be causing compatibility issues, or also be trying to open the pdfs.
Other third-party (non-Apple) apps eg: ad-blockers, virus checkers and the like ) can cause trouble.
or
could your eMail account app (eg Yahoo, Gmail etc.) need updating to be compatible with iOS 16.
or
sometimes simply re-starting an iPhone (or just an app) will solve a glitch.
The articles below depend upon opening the pdf attachment first, yet do show the correct procedure.
How to: Save PDFs to the Books app on your iPhone, iPad, or iPod touch - Apple Support
and
Apple support has articles online for iOS16
Read PDF documents in Books on iPhone – Apple Support
Download email attachments in Mail on iPhone – Apple Support
If needed:
Try the iPhone Support Team:
(There's a "Get Support" link (and a support app) at the bottom of their webpage)
iPhone - Official Apple Support
All the best :-)