If you want to ensure that your annotations are viewable on iPhone and iPad, you can save a flattened copy of the document.
1. Tap on the Document Overflow icon on the Action Bar. If you don't see the Action Bar, tap the center of your screen to bring it up.
2. Tap Save a Copy, then tap Flattened Copy from the sub-menu. The flattened copy's annotations are no longer editable but can be seen in any PDF viewer.
Creating a text markup annotation
Highlighting/text selection doesn't work on some documents
Saving a copy of a file
Saving a permanently cropped copy of a file
Saving a password protected copy of a file
Saving a reduced file size copy of a file