At 300 dpi 3000 pixels is only 10 inches wide. You'd need 150 dpi (dots per inch) at 100%. So: 24 inches = 3600 pixels. So a standard 2'x3' poster would have to be 3600x5400 pixels. A single image pdf is essentially just an image document, like a jpeg of tiff except it's made with Adobe Acrobat and is a nearly universal format that is read by the free acrobat reader. It's becoming a standard in the ppublishing industry. Kinkos can't just up and print a tiff or jpeg without first opening it in a graphics program and setting it up to be printed, hence the RIP charge. PDF's open right up and you just hit print.