Previously it was possible to copy text from Google books, but I think they had to give in to pressure from publishers (as google scanned millions of books without permission from publishers) and so from few months back it is not possible to do direct copy&paste, you have to manually retype it.

windows 7: use the mouse to highlight the text, press right mouse button, then select Oopy form the menu. Then open Notepad (or some other program) press right mouse button, then select Paste. Done. That method may not preserve formatting (line feeds and tabs).

Copying directly from the pdf-file (by highlighting the text you want to copy) isn't always a good idea tough..
I suggest you use Adobe Acrobat (more options than the free 'Reader'-software) to convert your pdf into a jpeg?
If that isn't possible (as probably is the case), I guess doing it the copy-paste way is the way to go, and re-assemble the lost layout using word.
Or you can go with print-screens, but that's really a last resort of course :)

as Peter_Budo already mentioned: pdf's can be protected in a way that highlight and copy-paste is no longer a possibility.
back to the OP: the possibilities depend on what pdf it is you are trying to copy.

or you can use pdfkey pro. that works good for copying PDFs also. that program will also allow you to edit / copy protected documents

Try using the "Snapshot" Tool which is available with PDF. You cannot do the same if you are using google docs.