When I import a PDF file to InDesgin the resolution from the PDF changes.
InDesign uses a low resolution thumbnail (might be the wrong way to descirbe, but easiest to visualise) to represent images/files placed into indesign. As InDesign is a page layout program it enhances the speed of the application as it doesn't have to draw the images in full detail, but rather just represent them in their size and position in the layout.
There are 3 viewing settings, available through the View> Display Performance (for the whole document) or Object>Display performance (on an object by object basis).
The resolution of the image from that point on relies on whether it was Vector or Raster, or in PDF and EPS cases, vector and raster. As already mentioned Vectors and PDFs don't contain a resolution. Vectors are mathematical equations that require the device outputting the image to draw them to the best of their abiltiy.
PDFs are a wrapper for a file, it can wrap text, vectors and raster for output.
But all the same, InDesign uses lo-res thumbnails to represent the PDF/Image/Object until ready to output. But you need to adjust the Display Performance.