how do I save a PDF attachment to iPad?

Assuming that you mean as a mail attachment ... Tap and hold down on the PDF icon and a window will pop up that will allow you to select Open in iBooks or any other PDF compatible app that you have on your iPad.

So if you have no app in which to save it, you can download iBooks or Adobe Reader which are both free and in which you can save PDF documents. There are many other apps that will open and save PDF files, search for more in the App Store.

Any suggestions on how to save a pdf attachment when using other mail apps, such as gmail Xfinity Connect?

I use the Xfinity Connect App myself with Comcast email and I see no way to save the attachment in the app. I can open the attachments, but there sure is no obvious way to save them.

Just double tap on the PDF and it"ll say "open in" at the bottom ....save in I books or documents or any such app u have on your I pad. Enjoy.