There could be multiple reason for pixelated image in PDF.
1) The link in broken for the image in InDesign. As you are seeing a good quality in InDesign then I guess that's not the case but worth checking.
2) PDF export settings, if you have down-sampled the image when exporting to PDF then that could be cause too. Please check the export settings or try to print from a pre-defined preset like "High-quality" to see if that's the reason.
bingo! i changed the settings when exporting and they look great now.
Thanks for your help.