Hi. You would just have to place a link to the image to your PDF. WHen they click on the image it would open the PDF or it can be downloaded.

There is a good plugin for adding custom fields in WordPress. You can add any type of data including pdf. It is the Advanced Custom Fields plugin, which can be downloaded from the wordpress site.

Do you want to make the PDF file downloadable for your visitors/customers?
If yes, and you want the image to be the "front" of it:
- upload BOTH files to yourserver/host
- make the image file a link (using anchor tag) to your PDF file

e.g. both file went up to your "uploads" directory, so

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It sounds like your link is getting put on an "attachment" page made for images. Are you using the "add an image" icon, or are you using the "add media" button in the editor? You should be using the "add media" button (spiky circle) for PDFs, docs and such.