If you are converting from PDF to PDF/A, use the PDF/A-1b format. As a rule of thumb, if given the option, save to PDF/A-1a from a non-PDF document (such as a Word document). PDF/A-1a has stricter compliance conditions, and is harder to convert to from a PDF. There are several different PDF/A formats, including PDF/A-1a and PDF/A-1b. Using PDF/A applies the necessary transformations to existing PDFs, such as embedding fonts, setting colour spaces, and a user interface for reading embedded annotations. PDF/A is specialized for digital preservation and archiving of electronic documents, such as theses, dissertations, and graduate projects.
