Actually PDF files do not have a single "DPI" value. Each bitmap object has a separate resolution, and of course vector objects such as text have no resolution at all.
To check the DPI for an image object, you can refer to the below.
- Open your file in PhantomPDF, go to Convert>Preflight.
- Under 'PDF analysis', select List page objects, grouped by type of object, then click on Analyze.
- The results will break your images out into handy ranges as the below.
- Open these out,you can get the dpi for each images.