I have thought this now for a while when using Indesign. I place a jpeg file within an indesign file and when I export it as a pdf I definitely loose quality. The images have not been scaled up - if anything I have reducing them in size on page?
Yes, absolutely — this is totally normal. That's how you get PDFs with such a small file size.
You can play around with image quality under the Compression tab in your export window. Experiment with both the Downsampling and Image Quality settings until you get the best balance between quality and file size for your requirements. Better quality = larger file.
With downsampling, as a rough guide, 72ppi = screen quality, 150ppi = inhouse print quality, 300ppi = press quality — try various numbers in between.