There is no way you should be getting PRN formatted files. Those are special printer language files that are not meant to be viewed by humans. You get a document, your program sends it to the printer as a PRN file, the printer reads the PRN file and does the commands inside to get you your image.
Read this http://www.pcmag.com/article2/0,2817,1945478,00.asp
You need to add a lot more details what you do to get these PRN files. If you are downloading coupons, they would come though as a regular image file from the web, not as a PRN file. What exactly happens with this PRN file, where do they come from.
Most coupon sites are just there to collect your email to resell to spammers anyway.