Obviously, if you reduce the size of the images, it will be more difficult to do a good OCR on the files. As far as the choice of PNG or JPEG goes, if you have Acrobat you can see whether you can use JBIG2 compression on JPEGs - it will decrease significantly the size of your pdf file - making it approximately as big as a text-based pdf.
If you can't do it with Acrobat, there is a program that can do this here:
Note that I use the program under Linux. I don't know how well the Windows version works.