If you're trying to extract the data (tables)
with the same (similar) structure (columns)
from the different PDF files (or the same file),
then UNIONing them in Prep looks like a solution.
But it would be a manual process (building a UNION).
To bring some sort of automation, one need to have data
in a suitable format. For Tableau (both Desktop and Prep)
a bunch of CSV files would be best to allow for a Wildcard.