When you create a PDF document your images are normally stored in a compressed format. The PDF creation process basically chooses between two compression types. Depending on which types of images you have in your PDF, you may prefer one over the other.
JPEG image compression
The JPEG compression is well suited for documents with photos. Images are compressed with a loss of information but it usually produces good pictures and reduces the size significantly.
FLATE image compression
In contrast to the JPEG compression the FLATE compression is lossless. The images are preserved and it does not introduce the noise pixels along sharp edges as you will find when JPEG compression is used. Therefore, this type of compression is preferred if you have images of diagrams or images containing text.
You can change the compression type, using different settings in the PDF configuration. If the Compress images is checked on the Document tab, then the JPEG compression is used. If you uncheck it, FLATE compression is used instead.