After digging deeper into old forum posts I still came up empty. So I thought
I'd describe the situation more specifically.

I tried a literal string...

-*/[a]*.pdf
+*/[c]*.pdf

I tried an escaped string...

-*/\[a\]*.pdf
+*/\[c\]*.pdf

I tried an escaped string within brackets...

-*/[\[]a[\]]*.pdf
+*/[\[]c[\]]*.pdf

I tried an escaped string within brackets preceded by an asterix...

-*/*[\[]a*[\]]*.pdf
+*/*[\[]c*[\]]*.pdf

If I'm understanding the rules correctly, either of the last two should have
done the job. But nothing worked so far. Then I noticed in the activity window
that HTTrack is converting square brackets into html escape codes. So I
thought what have I got to lose?
I tried...

+*/%5bc%5d*.pdf
-*/%5ba%5d*

... and it worked. Go figure, eh?