When you export an InDesign document to PDF with downsampling enabled, large images can require large amounts of memory. If the memory needs exceed the amount of system and virtual memory available, the export might fail with the message, "PDF Export Failed."
This is typically caused by placing images, or graphics that contain images, and scaling them down significantly. This causes their effective resolution to increase. For example, a 600 dpi image scaled to 50% has an effective resolution of 1200dpi. But placing a large, high resolution image without scaling could also require more than the available memory, depending on your system configuration and state, and the downsampling settings.