The Adobe CreatePDF app does not say it can convert the email itself to PDF, just the attachments you receive in the email.
I dont' have the app, but at a guess you just tap and hold on the attachment until the menu pops. Depending on your iOS version you will either get an Open In option or some icons. Either way the CreatePDF app should be a choice.
If the attachment displays inline, tap and hold as above.
If the attachment opens in Quick Look mode, tap once in the center of the screen (if the menu bar does not appear at the top), then choose Open In, or the arrow/box icon (depending on iOS version).
If all you need is the text of the email, you can tap and hold to select a bit of text in the email, then choose the Select All and Copy options from the popup menu. This can be pasted into any number of text apps, many of which can export as PDF.
If it's the saving of the email and not the PDF part that is important, you can use Evernote. An Evernote account comes with an email address. If you forward an email to that address it gets saved as a note, complete with most (most of the time) of the graphics and other HTML formating intact. You can even specify the notebook by adding @notebookname to the subject line.
I looked in the App Store and found the PDF Creator for iPad app. It says you can copy from email (described above) and past into the app to convert it to email. I have no idea if this will save more than the text, and the total lack of reviews means I have no idea if the app is worth buying; but it's the only thing I've seen that suggests you can turn an email (intact) into a PDF.