Would you press & hold over text to see if you can actually select the text?
If you do not see text selection with the context menu above (with Copy, Highlight, Strikethrough, Underline, and Define), it means that your PDF document is likely a scanned PDF.
It is not possible to add Text Markup annotations (Highlight, Strikethrough, and Underline) to a scanned PDF document because it does not contain actual text.
However, you can add Sticky Notes, Freehand Drawing (scribble), Text, or Signatures to a scanned PDF document.
Another possibility is that some of your PDF documents are protected or encrypted.
Please let us know if you have additional questions.
But I'm able to highlight the same scanned PDF on my Mac. It is just not possible on the iPad. Whats wrong there?
In Acrobat Reader for iPhone/iPad, do you see the lock icon in the bottom toolbar?
If so, your document may be encrypted due to document security or restriction settings.
Acrobat Reader mobile apps (for iOS, Android, Windows Phone) have the known limitation, where changes cannot be saved to encrypted PDF documents. Therefore, users are unable to make any changes (e.g. adding annotations or filling out forms) to encrypted PDF documents.
In contrast, there is no such limitation in the desktop version of Acrobat Reader, Pro, or Standard.
Sorry for the inconvenience.