Use storeToURL with the right filtername
sUrl="file:///myhome/documents/source.sxw"
oDocument=oDesktop.loadComponentFromURL(sUrl,"_blank",0,DimArray())
Dim args(0) as new com.sun.star.beans.PropertyValue
args(0).Name = "FilterName"
args(0).Value = "writer_pdf_Export"
destination = "file:///myhome/documents/test.pdf"
odocument.storeToURL(destination,args())
oDocument.close(true)