[RoyC]

no hurry! this post is not a complaint; only trying to understand the options.
Modifications of original scenario I have tried with identical results:
(a) moved folder from NAS to PC
(b) replaced spaces in PDF file names with "_" (underline)
© removed all spaces in file names
(d) file names all caps
(e) file names all lower case
this is not a file naming problem.

Since I can batch import to the internal SD card with no problem, this must be an external SD card problem and may be unique to my tablet. However, External SD card has no problem with other tablet software file operations.

I plan to try tablet external USB next.