The difference between 'a' and 'the' is always the same. It depends how many there are. If there is only one that you could take - it is 'the'. If there are several available, and you can get any one of them, it is 'a'. If you haven't printed it yet, it would also be 'a' - as you could print it again if you like (so there are potentially many available again) Whether or not to put a space in is personal preference, as it has (IMHO) become a compound word.

Printout = a compound word. meaning the the paper you printed off your computer or other printing device. Print out= meaning get the print out of the water or get the print out of the trash or get the print out to the printing company.