Enable file naming for single-page pdf

If we are only outputting a single document, the job file should allow
naming the pdf

Fixes https://gitlab.com/kicad/code/kicad/-/issues/20903

pcbnew/pcb_plotter.cpp

@@ -101,7 +101,10 @@ bool PCB_PLOTTER::Plot( const wxString& aOutputPath, const LSEQ& aLayersToPlot,
     LSEQ layersToPlot;
     LSEQ commonLayers;
 
-    if( aOutputPathIsSingle && !m_plotOpts.GetDXFMultiLayeredExportOption() )
+    const bool isPdfMultiPage =
+            ( m_plotOpts.GetFormat() == PLOT_FORMAT::PDF && m_plotOpts.m_PDFSingle );
+
+    if( aOutputPathIsSingle && !m_plotOpts.GetDXFMultiLayeredExportOption() && !isPdfMultiPage )
     {
         layersToPlot.push_back( aLayersToPlot[0] );