A Sales Order is not a transaction, so there is nothing to post.  When something ships, an invoice is created from the SO, and the invoice can then be posted.  When you have Paperless Office correctly configured, printing the Sales Journal (starting the posting process) will create the PDF.

When you say 'AR pdf log', if you mean the AR_CustomerPDFLog, when a Sales Order is printed to PDF with Paperless Office, a record for each document will get written to the AR_CustomerPDFLog table.

Thanks Kevin!  

Thanks Steve!