Archival PDF has certain codes stripped out of it so that it is more likely to open in years to come when newer versions of PDF are available. This is because old code is not always supported by later versions.
If you have a PDF writer you will need to follow the instructions for that particular version.
I am not aware of any mac version which allow direct saving to archival PDF.
If the document is already in PDF then you need a PDF writer.
When you want to save the word doc go to ‘save as’ as normal.
On the save as button go to the save as type option list and select PDF by clicking on it
Click on options
The following will appear
Make sure the iso19005 box ( sometimes call archival PDF/A, PDF/A or as in this example called both)
Is ticked and press ok.
The box will go and then press save and the doc will be saved,
Note
2 if PDF does not appear as an option for ‘save as type’ or there is no option in the PDF screen then I am afraid your word is too old to be able to do this and you will need to ask a colleague to do it for you.
Bob Barker
23/1/2019