How to find image compression type used in PDF?
The 'compression type' (e.g. JPG, PNG, TIFF…) can vary from image to image within the PDF document, which is why it's not shown in the general document properties. Instead, use the Preflight function to analyse all document pages and get a list of page objects including every single embedded image and its compression/encoding method.

Steps (based on Acrobat Pro XI):


In the right-hand Tools column, go to "Print Production" and click "Preflight".
In the Preflight dialog window, go to the tab "Profiles" and expand the list of profiles under "PDF Analysis".
Choose the profile "List page objects grouped by type of object", then click the 'Analyse' button and wait until the process ends.
In the 'Results' tab, expand the line "Overview", then "Images", then expand the image infos by page number and colour mode (CMYK, RGB, Grey/Black). Double-clicking the page-entry line highlights the image on the document page.