Please let us know the version of Acrobat Reader & iOS installed on your iPad?
Make sure you are performing Highlight operating on text, not on image.
Reboot iPad & then try to replicate the issue.
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. You would need the desktop version of Acrobat Pro (not Acrobat Reader) to apply optical character recognition (OCR) to your scanned PDF document and convert it to a fully editable copy.
Acrobat Reader and iOS versions are irrelevant in this case.
Please let us know if you have additional questions.
thanks a million it is a scanned document. Very helpful responses.