The most likely explanation for this is that you are viewing the PDF in an app that does not support PDF annotations. Many iPhone and iPad apps, including Mail and Safari, do not support PDF annotations. Apps that support PDF annotations include Xodo, Acrobat Reader, and Preview on OS X.

If you want to ensure that your annotations are viewable on iPhone and iPad, you can save a flattened copy of the document. This adds the annotations into the document as regular content that can no longer be edited.

1. Tap the overflow button in the command bar.

2. Select Save A ...

3. Select flattened copy