How can I create a PDF / A-compliant document?

That depends on the creation process and the programs involved. Either the source program (e.g. the invoicing program) that generates the invoice document can output PDF/A directly or only PDF, which then has to be converted to PDF/A using a converter. A possible converter would be eg intarsys PDF/A Live!. Another possibility is to use a PDF printer driver that can create PDF/A. To do this, the invoice must then be printed from the source program on the relevant PDF printer. Known solutions are eg PDFCreator.