You can check the resolution of a PDF image by using the Preflight tool. In this example, I'm working with Acrobat XI.
1. Go to Tools . . . Print Production . . . Preflight.
2. On the Profile tab, click open the PDF/X Compliance drop down menu and then choose Verify Compliance with PDF/X-1a. Then click 'Analyze'.
3. You will be moved to the 'Results' tab. Look in the Overview drop down menu. There will be a submenu for Images which will show the width and height for each page in pixels.
4. Pages of a different print sizes (8.5 x 11; 11 x 17, etc.) will have set pixel dimensions for standard resolutions such as 300 PPI. So a PDF letter size page with pixel dimensions of 2550 x 3300 will have a resolution of 300 PPI.
5. Alternatively, if you click on the magnifying glass icon on the toolbar of the Profiles tab, and then in the Image drop down menu select the option for 'Resolution of bitmap images between 300 and 600 dpi', you can Analyze the resolution of each page.
PDFs do not have a single resolution. There is a different resolution for each object in the file. The resolution of text and vector images will not change with magnification.