You are somewhat confused in that PDFs don't naturally have a resolution, and when you export the JPEG image from Preview, you specify a target resolution -- so the image resolution, by definition, can never go down. I suspect that you are complaining about the quality of the image the you get when exporting from Preview. There are two parameters that Preview allows you to set when exporting a PDF to JPEG. resolution and quality. Quality is a factor given over to the JPEG image compression algorithm and controls how accurately you want the picture to reflect the original. At the "Best" setting, you get minimal compression artifacts and blurring. If you set quality to the "Least" value, then you get an image that takes up very little disk space, but tends to be blurry with dulled colors, and obvious compression artifacts. Resolution defines how many pixels will be used to draw the output image. PDF documents have dimensions, such as 8.5" x 11" that defines the size of the image as it is intended to be show or printed. Since PDFs are mostly composed of vector graphics and text, the PDF has no native resolution. It's printed or drawn at whatever is native to the device. JPEG images, however, are sized in pixels (picture elements; the little colored squares that make up the image). In Preview, you specify the "resolution" of the JPEG in the export dialog by setting a number in the "Resolution" option, typically in "pixels/inch". 300 pixels per inch is the standard size for commercial printing. If you had an 8.5" x 11" page and saved it at 300 pixels per inch, you'd have a picture suitable for printing at 8.5" x 11" and it would be 2550x3300 pixels (8.4 megapixels). If you wanted to print the picture at 2x the size, you might select 600 pixles/inch, which would give you a 5100x6600 (33.7 megapixel) image suitable for printing at 17" x 22". OS X Preview does a fantastic job at rendering PDFs as JPEGs, provided that you set the JPEG quality and resolution parameters to something that meets your needs.
It should make your life more easy because you don't need to manually adjust the size, or change to something less than 300 pixels per inch to get those great looking JPEGs.