Word Interop is a nice solution, yet very slow (comparing to OpenXml) and unreliable (we encountered that if a user clicks Ctrl+C to copy something, whilst Interop is running in a background, generating multiple documents, its content will be pasted onto one of those documents, although the user intended to paste it somewhere else) . There are 3rd party solutions to save as PDF from OpenXml, yet they do not render correctly our relatively complex invoices (header with image and table, footer with table and so on), so for now we ended up to generate them all as docx and then run Word Interop from a server to convert them into pdf, using SaveAs2 method.