I have that sometimes. Typically there's an image somewhere that is causing the crash. Go to Window in the top menu and then down to utilities and background tasks to watch it PDF and find out which page is having the issue.
You'll probably have to do that page individually or re-save all the graphics on it.
It also helps to restart InDesign before you do this.
have they supplied the ads as a pdf? you just dropping their pdf into your document and then exporting?
i would do as has been said, watch it roll through the pages and try and find out where it crashes, isolate the problem
delete that page from document & export what is left
take the dodgy page and analyse it in indesign using package under file
try exporting it as a single page
try (if they have supplied a pdf) just dropping that into your pdf layout
if it still does not export i'd send it back to source and tell then there are issues with their file, let them export it as a single page pdf, send your pre-sets to them, if it is there problem they should sort it
then just drop it into other document
i have had issues where someone has set a file up with so many layers, vignettes and assorted other effects it just kept crashing
also in the past opened a dodgy pdf in illustrator, convert everything to outlines and cmyk and tried exporting that