Action Extensions have been around since iOS 8 (see here
for some further reading). They allow apps like ours to interact
with others. In the context of Papers, the action extension that we
have built allows you to import webpages and documents from other
applications like Safari or another PDF reader. Actually just about
anywhere you see the share icon
When you tap the share icon in another app you'll see two or three rows of icons. The last row in grey are your action extension.
If you do not see the "Add to Papers" icon, scroll to the right and tap on the "More" button.
Switch Add to Papers on.
From now on you'll be able to Add to Papers directly from that app using the action extension in the sharing menu.