When I check my email the attached PDF is always open.
When I check my email on my iPad, the attached PDF is already open. This has caused some problems
Thanks
Simon
iPad Air 5 Wi-Fi
See the last sentence of the first paragraph here. Although it is written in the context of an outgoing attachment, it applies to incoming attachments as well.
Add email attachments in Mail on iPad - Apple Support
The attachment size determines whether it appears inline with the text or not.