Hi,
If you're asking about sticky notes in Xamarin iOS app you could do a long press on text, select it and tap "TA" option from MenuItem and in Xamarin Android app you could add it using the note annotation button. To save the edited PDF you must put the PDF in an editable directory.
If you're asking about free text annotations you could implement in Objective C or Java code your custom methods. I suggest you to take a look at this method in PDFIOS.h:
bool Page_addAnnotEditbox( PDF_PAGE page, PDF_MATRIX matrix, const PDF_RECT *rect, int line_clr, float line_w, int fill_clr, float tsize, int text_clr );