How to Check the DPI of a PDF in Adobe
The DPI of an image is its resolution, or its pixels per inch. You might need to adjust the DPI of a PDF file for higher or lower resolution depending on your needs. To make the file size smaller, you can reduce the DPI. Before you make adjustments, however, you need to know the PDF’s current DPI.
Open the PDF in Adobe Reader or Acrobat.
Find the “Image” menu at the top of the window and then select “Image Size.” A new window will open, displaying the “Pixel Dimensions” and “Print Size” of the image.
Make sure “pixels/inch” (rather than, for example, “pixels/mm”) is selected in the “Resolution” drop-down menu. The number displayed next to "Resolution" is the Image’s DPI.