PDF is designed to be generally unalterable, as is the page of a printed book. If you select text within the pdf file you may be able to copy and paste it to an OpenOffice file, provided the pdf author permits the copying when generating the pdf file. Otherwise you may need to use an OCR program to translate the pdf file, which is a picture of text, back to individual characters.
I think that You will need an extra application like Adobe Acrobat. Converting PDF to Word is not so easy (on any OS). I tried this once on a Windows PC, but the result was so bad, that I gave up. I don't know how many documents You have and what You need them for. But sometimes it is just quicker to just copy the text from the PDF and to paste it in a word editor and to edit it there.
Other solition: read this: how to convert PDF to MS word on Mac os