Views link image to field
You may be able to use "Rewrite the output of this field" to wrap the image in a link using the "replacement pattern" token for the PDF file url as the href there.

Note you can "Exclude field from display" to add a field to the token list without displaying it.

You'll also need to drag the field, in the field order, above the one you want to use it in as a token for it to be available.

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