It depends what you're looking for by your word, "attribute".
Using Access Services may incur more than one call; or you may be able to get it using a view.
A view might help, it'll get you the column values in the view, for each document.
It looks like there's a "View/Folder Entries GET" request that could fill in for multiple docs.
I haven't worked with Domino Access Services itself, but I'm familiar with the older CGI requests that could easily get you the UNID & certain field values. They'd just be in view columns.
http://site/dbpath/viewname?ReadViewEntries&outputformat=JSON
There's a limit on the count of row entries you can retrieve. I think there's a way to kick it up to something like 999 rows, but I know there's a limit.