There are a few ways you can work with PDFs in your native Mail app. When you open the email, you should see the PDF attachment.
When you tap on it, the PDF should open right up in your Mail app. If you tap Apple’s Share function, you will get a list of things you can do with that PDF! If AirDrop or AirPrint are enabled for your device, you can share it that way.
There is also a list of other apps that you can use to share your PDF. My first four are Message, Mail, Open in iBooks, or Open in Skitch (an app I use to write on my screenshots for writing these tips!)
If you scroll over, you may have more options, like Chrome and Google Drive for me. Tap More to see other options. From this More view, you can add options that may be disabled, or customize the order of your share options.
Top image credit: Sergey Nivens / Shutterstock.com