Re: How do I save Illustrator CS3 file as 300 DPI pdf file?

If you want to keep it vector art just go to SAve as and choose pdf and leave everything clicked on in the options. Vector art doen't have a dpi becuase it's basedon mathematical points, called vectors which means it can be scaled up or down with loss in detail or resolution.
If you rasterize it that's a different story. Then you can scale down without out too much of a problem but you really don't want to scale up because then your image will get soft and fuzzy or pixelated.
You can also Export your file as a Photoshop file by going to File Export, then pick psd file. That's when you can pick 300 pixels per inch. After that you could open the psd file in Photoshop, flatten if need be, and then save that as a pdf as well.
There's more than one way to do it.