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.
1.Open your file in Foxit PDF Editor, go to Convert>Preflight.
2.Under 'PDF analysis', select List page objects, grouped by type of object, then click on Analyze.
3.The results will break your images out into handy ranges as the below.
4.Open these out,you can get the dpi for each images.