Hi Mehregan,
You upload the PDF into your Media library:
0_1499689034021_Bildschirmfoto 2017-07-10 um 14.16.32.png
Then you click on the PDF and copy the url on the left:
0_1499689062604_Bildschirmfoto 2017-07-10 um 14.16.37.png
Then you go the the gridder and place an image
Then you right-click on the image and add the url under LINK IMAGE:
0_1499689108268_Bildschirmfoto 2017-07-10 um 14.16.14.png
I hope I could help!
Many wishes!
Marius