We are looking for a solution to generate documents in PDF/A format for sharing and also archiving purpose.
I checked the description of ExportFormatType.PortableDocFormat, however it just say PDF file.
Can the Crystal Reports generate PDF/A compatible files?
According to this SAP community thread from a few days ago, it can't be done natively, although there was a third-party component mentioned there. I haven't tried it, so I have no idea if it works or not.
I don't think you export directly to PDF/A. Instead, I recommend using Crystal to export to PDF, then find a third-party software to convert your PDF to PDF/A. It takes 1 extra step, but it will meet your needs.
I googled a bit and found http://www.abbyyusa.com/shop/pdftransformer/. I know nothing about this software, I'm just presenting it as an example. It costs 80USD, but you might be able to find a freeware alternative.
http://www.pdfa.org/doku.php is the offical homepage of PDF/A. You might find something useful there too.