Disclaimer: I am not well versed in the subject of PDFs, their different flavours, and how to convert to them. I hope someone with more knowledge comes along to help.
Are you looking for a standalone, out-of-the-box software product, or an SDK to integrate into your own application? If you're looking for standalone software, I believe Word allows you to save a document as a PDF with the option to make it PDF/A-1 compliant, but it doesn't specify if it is 1a or 1b.
If Word is not sufficient for your needs, there are two other products that I can point you at. Both are commercial products that are available as either a standalone product or an SDK. When I was looking at these products, I was looking for other features, so
I am not certain if these will fit your needs. If I recall correctly, both of them allowed Word docs as input format and some form of PDF/A as output format for a conversion. You may need to contact the vendors to confirm that they support the formats you are interested in, if you cannot find that info by digging around on their websites.
These products are:
PdfCompressor[
^] by CVision and
IRISCompressor[
^] by I.R.I.S. (or
iDRS[
^] if you prefer an SDK).
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