The quality level of JPG images during conversion can be controlled using the SetJPEGQuality function. A number between 1 and 100 indicating the quality of the image. The higher the value, the better the image quality, but the larger the file size. The lower the value, the smaller the resulting file size, but at the expense of picture quality.
The default is set to 75 and this value will be used if you do not call the SetJPEGQuality function.
See Convert an image to a PDF programmatically for information on how to convert images to PDFs using Foxit Quick PDF Library.