I think you mean PPI (pixels per inch). DPI is a printer term, PPI is used to measure resolution.
That said, when you export to PDF you are using an Acrobat job option (called a Preset in FrameMaker) to control the output settings. If you choose Smallest File Size, Standard or Standard Classic, the images will be downsampled (the resolution will be reduced) on export. Edit the .sts file in the Publish console and choose one of the presets that does not compress the images.