FYI, it's not "3 dots/periods" - it's an ellipsis, inserted by pressing "Option-semicolon" on your keyboard. I got my tech writing degree in 1988, so that's how I knew what an ellipsis is - it represents an implied "there's more to follow"…
System Preferences > Keyboard > Shortcuts Tab > App Shortcuts item > click "+" > choose "Preview" from the Applications menu > type in the menu item exactly as you see it - Export… (or Export then Option-semicolon, with no spaces) > and type in your preferred Keyboard Shortcut > Press the Add button.
You're done - you don't even need to restart or log out.
Make sure you check for Command conflicts. Keep in mind that some applications don't work properly, like Firefox, which only responds to its own built-in commands - Shortcuts may show up but won't work and you'd need to resort to using your mouse or built-in command structure.
FYI, you can also use System Commands. I use "Command-P" in "All Applications" to invoke the Print Dialog, and while in that Print Dialog Box press my Keyboard Shortcut "Command-P" to invoke the "Save as PDF…" Command that's in the pull-down menu. So, you're not just limited to the visible commands.