Hi Christofer,
PdfReader.unethicalreading = true is C# code, so you would need to edit the extension via OutSystems Integration Studio and add that line of code on the right spot. If it needs to be optionally set you would have to add in to the extension method JoinPDF a boolean input parameter that you can use in the extension code to set the PDFReader.unethicalreading variable.
Line 344 is probably the line where you should add it, the JoinFiles method is used by the JoinPDF action

Regards,
Daniel