If you want to export your Word document on a Mac to PDF/A-1a, then get the free LibreOffice, and once your document is opened, choose File menu : Export as PDF, and on that export panel, there is a checkbox for PDF/A-1a. LibreOffice is an MS Office replacement suite.
If you subsequently open that LibreOffice generated commercial PDF in Adobe Acrobat Reader 11.0.14, or version 15, the following banner will appear. Click to enlarge.
I would be very surprised that MS Windows, or MS Word on Windows is now exporting PDF directly to PDF/A-1a standards without the help of Adobe Acrobat Pro's Writer, as this would have to satisfy several PDF/A-1a criteria found in that specification.
The native document format on OS X is PDF v1.3 via either an application's File menu : Export, or File menu : Print : PDF. That version of PDF implementation cannot conform to the minimum PDF v1.4, or 1.7 that the PDF/A standards are based upon. Partly due to this issue, Apple does not ship any OS X applications, or other OS X applications that generate the PDF/A output. Third-party application vendors that wish to implement their own PDF export/printing solution can and do implement the PDF/A standard as they have no operating system restrictions.