Question

Is there a way to scale the output PDF file to the specified format?

Answer

Yes, to achieve the desired behavior, you need to specify the properties of the PaperSizeParams object and pass it along with PDFExportParams to the Export method. For, example:

// C#
PDFExportParams pdfParams = Engine.CreatePDFExportParams();
PDFExportFeatures pdfExportFeatures = pdfParams.PDFFeatures;
 
// A4 format, size specified in twips
pdfExportFeatures.PaperSizeParams.PaperHeight = 11909;
pdfExportFeatures.PaperSizeParams.PaperWidth = 16834;
pdfExportFeatures.PaperSizeParams.PaperSizeMode = FREngine.PaperSizeModeEnum.PSM_UserDefined;

document.Export( pdfExportPath, FREngine.FileExportFormatEnum.FEF_PDF, pdfParams );

You can find more information in the Developer's Help > ABBYY FineReader Engine > API Reference > Parameter Objects > Export Parameters > PaperSizeParams.