Are you running iOS 6? Things are slightly different between iOS 5 and 6, though, in general, things are in the same place.
When the PDF file is open in Mail (viewed full screen), do not tap and hold. Just tap in the center of the screen. The share icon (a square with an arrow coming out of it) should be at the top right. Tap this and you'll get several icons. Some are system options like Copy and Print. Others should be your PDF Reader apps and any other PDF compatible apps you have. In iOS 5, you should get a list of options instead of the icons. One of those options should be Open In.
Alternately, if you tap and hold on the PDF in the email's body, you should get the same list of icons and/or options.
If your PDF apps are not showing up, please let us know what apps they are. It's up to the developer's to enable their apps for Open In with whatever files they are compatible with. All the popular apps (that I know of) work fine: Adobe Reader, GoodReader, iBooks, PDF Expert, and many more.
Also, in iOS 6, the list of icons that show up make take up more than one page. You can swipe through multiple pages of icons just like you do on the main screen.
Because it hasn't been mentioned yet, I'll include a list of common fixes below. If the problem persists and you have not tried them yet, please do. It's a good idea to restart your iPad every so often anyway.
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The usual fixes:
When you have problems with an app it's a good idea to clear it from RAM.
Close/Clear the app: With the problem app closed, double tap on the Home button to get the multitask bar. Tap and hold on any icon in the bar until they jiggle, then tap the minus sign on the problem app. This will remove it from RAM and insure a clean start the next time you launch the app.
If that does not work try a restart,
Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears. Apple recommends you restart the iPad at least once a month.
and then a reset if that does not work.
Reset: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart.
Reset All Settings: Another, somewhat more drastic thing you can do is Reset All Settings. You'll find this under Settings > General > Reset > Reset All Settings. It won't erase any of your content, apps, or accounts; but you will have to go through most of your other settings and re-tweak them back to your liking.