Looking at the source code for iTextSharp, it's not GetPdfVersion you should be calling, but just PdfVersion, which is a property of the PdfReader object, not a method.
using (PdfReader reader = new PdfReader(filePath))
{
string pdfVersion = reader.PdfVersion;
pdfVersion = pdfVersion.Replace("PDF-", "");
return string.Compare(pdfVersion, pdfStandard) >= 0;
}