There is a known conflict between Finale v27 and Microsoft Print to PDF, the default PDF printer on Windows, that will not allow PDF exports for Finale files with custom page sizes. In order to export PDFs with custom page sizes, you'll need to use one of these two workarounds:
Third-party PDF Printer
Third-party PDF printers, such as CutePDF and Win2PDF, will allow you to select custom page sizes in their settings before exporting PDFs from Finale.
Exporting a TIFF File and converting to PDF
Export a TIFF file from Finale using the Graphics Tool
and use a third-party tool to convert the TIFF document into a PDF. Adobe has a online tool that you can use for that conversion process here: Adobe Acrobat Image to PDF Converter.