Can I use the 'Speak Screen' option to have document text read by Siri?

Yes you can.  To do this, go to the iPad's Settings app and select GENERAL > ACCESSIBILITY and make sure SPEAK SCREEN is turned ON.  

Now, open ForeFlight Mobile and go to the Documents page and open a document.  Once open, swipe with two fingers from the top of the iPad screen to the bottom.  Siri will start reading the text and a control panel will be displayed.  

After about 10 seconds, the control panel will minimize to the left side.  You can access the controls by tapping on the tab.  Selecting the (X) will stop the reading and close the Speak Screen feature.

Image 1:  Speak Screen control panel after 2 finger swipe down.

 

Image 2:  Minimized Speak Screen controls on left.