The goal is to restrict direct browser access to all *.pdf various subfolders within a main folder. The subfolder names in which each pdf file reside are numeric only (characters 0-9; Eg. "work/books/112/myfile.pdf")
What I have so far is:
RewriteRule ^work/books/([0-9]+)/\.pdf)$ [F,L,NC]
Where 'books' is my main folder and then whatever numeric folders in which pdf files reside, there are.
This doesn't seem to work and I am also unsure if while this rewrite rule prevents direct access of those pdf files, will I still be able to read the pdf files via php?
The rule is missing the file's name part before the extension (.pdf), so the regular expression should be
^work/books/[0-9]+/[^/]+\.pdf
There's also no need for capturing any part, this means you may omit all the parenthesis, which are unbalanced in your example, btw.
Another part of a RewriteRule directive is the substitution, which may be just a dash in this case. So putting all together, it becomes
RewriteRule ^work/books/[0-9]+/[^/]+\.pdf$ - [F]