The addMargins method in com.aspose.pdf.facades.PdfFileEditor Class is defined as below as per Aspose JAVA Doc:
addMargins(InputStream source,OutputStream destination,int pages, double leftMargin, double rightMargin, double topMargin,double bottomMargin)
It seems to me that value given for topMargin is actually being used for bottomMargin and vice-versa. I noticed during one of my example program. You too can write a small example program and test it.
Could you please check this issue?