Saving an image file in the JPEG format is a commonly used method of “lossy” compression for digital photographic images. The degree of compression can be adjusted, allowing for a user selectable tradeoff between storage size and image quality. The greater the image compression the smaller the resulting image file and the greater the loss of image quality.
By default, images are JPEG compressed when saved as a PDF file.
How JPEG compression works
See full article at: Quality In Print: JPEG images for print production – the facts.
