CUPS-PDF normally creates a directory ~/PDF in which to save the PDFs created. I want to change
the default save directory instead to ~/Downloads/PDF. I'm running Devuan Jessie.

The first problem is that my spool directory is /var/spool/cups-pdf has two subdirectories,
ANONYMOUS and SPOOL rather than a username subdirectory. Which of these is really the cups-pdf
spool directory?

To redirect PDF spool files, I stole a script:

#!/bin/sh
SPOOL_DIR=/var/spool/cups-pdf/
USERS_DIR=/home/haines/Downloads/PDF/

inotifywait -mq --timefmt '%Y%m%d %H%M%S' --format '%T %f' \
-e close_write $SPOOL_DIR \
| while read date time file; do
mv $SPOOL_DIR${file} $USERS_DIR${date}${time}_${file}
done

When run it produces the error:

11: /home/haines/scripts/cups-pdf: inotifywait: not found

The default output directory is /var/spool/cups-pdf/user. You can change the output directory in the cups-pdf.conf file to ${HOME}/Downloads/PDF without needing to use inotify.

Normally you would need to install the inotify tools package to use inotifywait. Not sure if Devuan is the same.

I wanted mine accessible from ~/Documents so I did a symlink:

You could do the same with Downloads:

EDIT: for anyone who is brand-spanking new to Linux or symlinks, a symlink is a symbolic link, or a way to tell the filesystem that you want it to treat a certain location as if it exists in another location as well. There are different kinds of symlinks, the one above is a soft symlink. Windows can do this as well with the mklink command.

What the commands above did:
1) change directory (cd) to the Documents (or Downloads) folder in the home folder(~)(/home/yourusername)
2) create a soft link to the folder PDF in the parent folder of the current folder (../) which is ~/PDF, and name the soft link "PDF Printed" in the first example or "PDF" in the second - keep in mind that paths with spaces must be surrounded by quotation marks "". (./ is the current folder, ../ is the parent, ../../ would be the folder above that, etc).

-Why not just link to the absolute path (~/PDF or /home/username/PDF) instead of the relative path? Well, you are unlikely to move your home folders but let's say you did, or you copied them to an external drive under /myprofilebackup/ or somesuch. A link to ~/PDF will point to the PDF folder in the current user on the currently running system's home folder, and a link to /home/yourprofilename/PDF will not work unless that path exists (if you're on another computer borrowing a login, say), whereas a link to ../PDF will point you where you want to go, so long as you also copied the PDF folder along with the folder that has the link to it.

More info on Linux hard and soft links
More info on NTFS Soft and Hard links and Junctions