The Xeroxes are normally set with a default resolution of 300dpi, which is a good compromise between output quality and file-size. You can alter this either up or down, depending on the complexity or detail of the original – from the Email screen, choose the Advanced Settings tab and then the Resolution option. Bear in mind the higher the quality (max. 600dpi) the larger the resulting file-size, so you may run into problems transferring the file to the network if the resolution is set too high.
If you used the document feeder (ADF) and your PDF is produced with very fine lines across it of uniform thickness, then the scan window needs cleaning. When scanning through the ADF, the Xerox uses the letterbox scan-window to the far left of the main scan glass, and any debris or specks of dust on this will cause streaks across the PDF output (which will not be present if you scan the original off the main glass). Sometimes the debris isn’t visible, but you’ll be able to feel it if you run your fingernail up the glass. Simply clean the glass strip and re-scan.