Hi and welcome to the forum!
Is the PDF a scan with a big picture of a text? If so, no way to get the text (except with an OCR, but that's another story).
NB: so many topics about that, don't forget to search the forum first.
AOO 4.1.3 on Xubuntu 17.04, (Zesty Zapus) and on Windows 7 (with winPenPack port).