Hi @KaKaRoT
Insted of just adding the PDF thumbnail, you will need to create an image CTA in HubSpot.
Here is step by step guide in how to do that:
https://scribehow.com/embed/Create_Image_CTA_in_HubSpot__gqXMk1N9RsmhmfUYDEVo0A
Once the image CTA has been created, you then add that CTA in your landing page or website.
https://scribehow.com/embed/Add_image_CTA_to_HubSpot_Landing_Page__g2G5ajQhSI6v-FGAfms8Qg
Once that is done, you can start building a list of contacts who have clicked that CTA.
https://scribehow.com/embed/List_of_contacts_who_clicked_a_CTA__aYC-j7GcR8ijZnwqZJjPjw
Once the CTA has been created and added, you can also check the built in HubSpot CTA reporting. Personally, I think a list is a good foundation, as it can be used for list performance reporting, an inclusion or exclusion list in furture workflows, custom reporting in HubSpot and smart content. If you want to find out more about lists and why they are pretty useful, check out the AMA I am hosting in the Community this week.
I hope the above helps.
Frank