A PDF is not an image, so uploading one into the html module is not the same as adding an image. This process is for when you want users to see the actual contents of the PDF inside the html module.

To add a PDF as an image, first open the PDF on your computer. Then go to 'Save As' or 'Export'. Near the bottom you will see your options available to save the PDF as other files types. Save the PDF as a PNG, JPG, or JPEG.

Then follow these steps for adding an image to the html editor - http://help.connectingmembers.com/category-2/html-module-add-an-image-to-an-html-module