One of the problems of going from an ‘image’ to a PDF, is that the file generally gets larger. PDFs store images as compressed rasters. PDFs then have to ‘wrap’ the image with the PDF encapsulation. As a result, PDFs do not make the image smaller– but do make it more portable. The exception to the PDFs make it bigger, is when the image that is generated by your software is not compressed (applies to some TIFF and BMP generators – though some may do a RLE type compression).

JPGs are compressed by ‘definition’. A PDF of a JPG is always going to be bigger.

As ‘flu’ noted, pay attention to what resolution you need and how much detail in the color. Some scanners will allow you to generate a ‘fax’ format (1 bit per pixel with primitive RLE-Run Length Encoding). This will tend to be the smallest, and is useful for sending documents – but not images. Another format to consider is also PNG.

Another way to create ‘PDFs’ is to use PDFCreator, which is a printer driver that looks like a printer but creates PDFs instead of printing.