Thanks for sharing the details.
I have tested the scenario using following code snippet and as per my observations, the pages in resultant file are in both Landscape and Portrait orientation (as per input files). Can you please share your sample files (save the blob to files), so that we can further look into this matter. We are sorry for this inconvenience.
FileStream fs1 = new
FileStream fs2 = new
// Open the first document
Document pdfDocument1 = new Document(fs1);
// Open the second document
Document pdfDocument2 = new Document(fs2);
// Add pages of second document to the
// Save concatenated output file