Hi jamess_10,
In general, you should be able to select text and add text markup annotations (Highlight, Underline, and Strikethrough) to a PDF document in Acrobat Reader for iPhone and iPad except the following cases.
(a) Your PDF document is a scanned document that contains no actual text.
You need to convert text images to editable text using OCR (Optical Character Recognition) technology.
Convert PDF to instantly editable text, OCR software | Adobe Acrobat DC
Please note that the free version of Acrobat Reader mobile and desktop apps do not support OCR.
(b) Your PDF document is encrypted. Acrobat Reader for iPhone and iPad does not support saving changes to encrypted PDF documents. This is a limitation in Acrobat Reader mobile apps for iOS and Android.
(c) Your PDF document disallows commenting due to document restrictions (set by the author).
Do you see the lock icon in the bottom bar when you open your PDF document in Acrobat Reader for iPhone and iPad? If your case is (b), you can still add Highlight to the document in the desktop version of Acrobat Reader.
Hope this information helps.