Under System Preferences » Keyboard » Keyboard Shortcuts, you can create a new Application Shortcut for all applications.
Call it "Mail PDF", and assign a custom keyboard shortcut, like ⌥⇧⌘P.
It is now available in the Print dialog:
To use it, press ⌘P from any application, then ⌥⇧⌘P. The only problem with this approach is that the "PDF" menu needs to be selected. I've found this to work inconsistently, so have others.
For another approach, open Automator.app, create a new Service. Set it to receive "no input" and add the "Run AppleScript" action from the library to the right panel.
Paste the following:
tell application "System Events"
set theName to name of the first process whose frontmost is true
tell process theName
click menu item "Print…" of menu 1 of menu bar item "File" of menu bar 1
click menu button "PDF" of sheet 1 of window 1
click menu item "Mail PDF" of menu 1 of menu button "PDF" of sheet 1 of window 1
Save it, then go to System Preferences » Keyboard » Keyboard Shortcuts, and go to Services. Here, assign a keyboard shortcut for your new service.
This works for most applications, but not all of them. I might find a way to debug why it fails, but I guess it depends on how the print dialog is implemented.