Within the bottom toolbar of the PDF document, tap the annotation sharing button.
The annotation sharing options view will be presented. Tap the second tab, titled "Shared with You." You will see a list of users that have chose to share annotations with you.
After tapping a user, their annotations will be displayed within the document and it will automatically scroll to the first annotation for your convenience.
Note that during this time, you cannot edit or create annotations since the document is in read-only mode. To dismiss the user's shared annotations, tap his or her portrait in the bottom left of the screen.