When you open a PDF attachment from your email then it opens up in Apple's built in PDF viewer.
In order to open it in Acrobat Reader you have to Tap Share icon>Then choose Copy to Acrobat.
For detailed information please refer to the help document for the same- Acrobat Reader Help for iOS: Opening PDFs in Acrobat Reader
Unfortunately, Apple does not provide a mechanism for iOS users to set a default app for a given file type (examples: .pdf, .docx) on iPhone/iPad.
You just need to continue using "Open in" or "Copy to" to open PDF documents in Acrobat Reader on iOS.
Sorry for the inconvenience.
It's 2019 as I'm posting on this thread. Ive been affected by this problem for a while but I think i have found a solution. I'm on ios 13 btw. Heres the steps. I deleted all pdf tools app such as acrobat reader, signpdf pro, etc from the phone so that mail is forced to use its native pdf viewer to open a pdf attachment within itself (as it used too😉). You may need to disable the mail (on off green button) under setting>passwords & accounts and restart your phone. After restarting go back to the previous steps by enabling mail by clicking the green button. hooe this works for you.