I am using a temporary licence evaluation copy of Aspose with .NET C#.
Tried to open a valid PDF file using below source code. fileInfo.IsPdfFile value was showing as false and fileInfo.IsEncrypted as true. See the attached image for exception details from watch window.
using (PdfFileInfo fileInfo = new PdfFileInfo(pdfFile))
{
boolIsValidFile = fileInfo.IsPdfFile;
boolIsSecuredFile = fileInfo.IsEncrypted;
}