Souvenir Program

Okay, I think you've got some confusion here. DPI and PPI, for the purposes of what you are doing, are the same thing- 300 DPI is the same as 300 PPI in an image editor such as Photoshop/ PSE etc.
PPI = Pixels Per Inch- this stems from the fact that digital images are comprised of pixels
DPI = Dots Per Inch- is a term that is used mostly in printing. Put simply (although there is much more to it than this, I just want to keep things simple here) printers output the pixels as dots of ink, the more ink dots there are in each inch of printed image the less obviously noticeable the individual ink dots are, print colours and blacks are richer, image detail is finer and cleaner. Newsprint for example is usually 200 DPI or even lower and you can see the difference between this and a quality 300 DPI (or better) magazine. Most images/photos on the web will be around 72- 96 PPI which looks fine on a monitor, but this resolution is WAY too low for printing. If your ad is just text, clipart, lineart or some form of vector graphic then just scaling it up to fit a 8” x 5.125" artboard in Illustrator, select any text and choose "Create Outlines" from the "Text" menu then save your ad as a PDF using the high quality print or press quality PDF presets option you will get when you "save as...". Things like photo images are raster based and these do not fare well with being resized or resampled to a higher resolution and the overall image quality will deteriorate- usually becoming 'fuzzy' and pixelated. So ideally you want to begin with a source image/photo as close to 300 PPI (or DPI) or greater (downsizing and downsampling a larger image to the target resolution is perfectly okay to do) and use the "Document Raster Effects Settings" in Illustrator's "Effect" menu to check the resolution and convert the photo/s etc to greyscale colour model.
I hope this helps you but if you are still stuck you are welcome to flick me an email via my profile and I'll be happy to help you further.