From a purely production standpoint the answer depends on the job. Ask your printer for the line screen that will be used. Then double that number. Most printers will even tell you the dots per inch as well. 300 dpi is just a cleaner number than 266 dpi (2 x 133 lpi), which is why it is used. It also comes from the fact that Art Directors, Account Managers and clients will ask for images to be enlarged at the last moment and you need to have enough resolution to accommodate that request without having to rescan.
The fact is that 300 dpi in Quark refers to 300 dpi at actual (100%) size. So, if the image placed in Quark is reduced, you can technically resize your image and create a smaller file. Remember that quark keeps track of all the images placed in it and increases the overall file size accordingly. I recommend that if you plan to resize your images, that you keep a copy of the original.
PDF from quark is done in Quark 5 and 6 by exporting the file as a PDF. You’ll find that feature under the file menu. In earlier versions you can get a free extension or save the file as a postscript file and run it through Adobe Distiller. However, in all versions, you really need a full version of Adobe Acrobat with Distiller to allow yourself the full functions for your particular flavor of PDF.