To sum it up for vb.net users, the free option (must have office installed):
Microsoft office assembies download:
Add reference to Microsoft.Office.Interop.Word.Application
Add using or import (vb.net) statement to Microsoft.Office.Interop.Word.Application
VB.NET example:
Dim word As Application = New Application()
Dim doc As Document = word.Documents.Open("c:\document.docx")
doc.Activate()
doc.SaveAs2("c:\document.pdf", WdSaveFormat.wdFormatPDF)
doc.Close()