The only way to export .dsn schematics from OrCAD PCB Designer as .pdf files, is to open them in the original software or some other supported viewer and print them as PDF using a virtual PDF printer installed, or via a dedicated PDF export function.
Unfortunately, it t seems there is no default PDF export function in OrCAD, so using the print method is likely the only way to convert dsn to pdf.
Updated: June 30, 2023