How to get the actual PDF icon rather than the stock one
Here is the two ways to apply an icon to a file.


  Note: To get a better view of any image shown below, click on the image or open the image in a new window.



Start with the Get Info windows for a PDF file and PNG file, as shown below.



To copy the current icon from the PNG file to the PDF file, click on the PNG file icon, as shown below.



Drag the icon and drop on the current icon for the PDF file, as shown below.



The result is shown below.


To change the PDF icon to be the preview image of the PNG file, create an image of what is shown in the Get Info window. To do this, use the Grab application to capture the selected part of the screen containing the preview. This will create the Grab window shown below. Look for the windows titled Untitled 2 - Edited. 



To make this image the icon for the  PDF file, click on the image, as shown below.



Drag the image and drop on the current icon for the PDF file, as shown below.



The result is shown below.