It is going to depend how large the catalog is too, a small 3x5 catalog should be scanned higher..
The 'theoretical' resoloution of the screen is 72DPI , so anything over that should be readable at least. (720 pixels on a 10 inch wide screen...))
pdf (really JPEG) compression is going to throw a lot away above 300 dpi regardless, unless you make gigantic PDF's..