Technotigers link is excellent - but if you have an existing PDF and want to shrink it dramatically without any "extra" software - it ain't possible.
What software did you produce the pdf with? - and what other pdf software have you got?
I make my pdfs with (free) Primo pdf maker, and that allows you to specify various parameters when you create the file.
All that being said - 25MB to under 7MB is an awfully big change - why do you need to do it?
I just did a quick test like this - took a single page pdf at around 2MB, displayed it on the screen (the whole of it) and did a screen grab. Result a jpg at around 25% the file size (a bit more shrinkage than you are asking for). Does that do what you need? - it will need a screen grabber though.