What is the "meaning" of the returned int from CAcroPDDoc.Open(string s) ???

I am updating to a newer version of Acrobat, and I have the following C# code which is giving me an error saying "cannot implicitly convert from int to boolean":

var pdfDoc = (CAcroPDDoc)Microsoft.VisualBasic.Interaction.CreateObject("AcroExch.PDDoc", "");

if (!pdfDoc.Open(SourceFilePath)) // <-- red squiggly under "Open()"

{

    throw new FileNotFoundException();

}

Obviously, in a previous version the Open() method returned a boolean value - now it returns an int.  Can someone please provide me a reference to this method so I can determine the "meaning" of the returned int values?  I cannot find any reference to this method.  Help!!