PDF/A is a file format not a protocol, so a pipeline component rather than an adapter would be a better option.
There is an example PDF generator pipeline component on GotDotNet
http://www.gotdotnet.com/Community/UserSamples/Details.aspx?SampleGuid=78C0FD43-E4C3-4DCF-AF23-FE72F317891A
This shows how to convert an XSL-FO document into a PDF in the pipeline. There is an example mapping a UBL Invoice into XSL-FO and being converted into PDF.
If this is not exactly what you need it may be a good starting point.