First of all, it appears that your PDF file was made from a file in which the text had already been turned into an image of some kind. I believe that that's the reason that you cannot copy it and paste is as text, presuming that you have tried the Paste Special/Unformatted text command. In this case, scanning to OCR is your best bet, as the other poster suggested.

As far as what size file Word can open, other factors, such as your computer's available memory, speed, and so on, effectively play a role. I have grimaced when seeing Word files of tens of megabytes, because they are unwieldy. Usually, the size is due to images contained therein, and there are ways of handling this (e.g., making images appear as placeholders, a setting in Word options), but, from your description, this is not going to help you.