I found a workaround of sorts. It takes more steps, though.
First, open the Search & Outline sidebar, and enter your search term. Then, click Select and All. Next, on the floating control bar, select … > Export to other Mindmap Notebook. You will be prompted for the name of a new notebook. Finally, open the new notebook that you just created. It will contain only the notecards you found in the search, linked to the PDF. You can now export this.
Data exporting is the primary concern. We are looking for testers who got experiences with AppleScript. And help us check what the API could do or optimize. Please click the link to join: Extstester - Marginnote