On Mon, 30 Nov 2020 13:39:01 +0100, Michael Logies wrote:

Assuming that the printer can "understand" the PDF format...
o Is this how an Android print to that printer is "supposed" to work?
1. The Android editing app reads the file in a native format (e.g., JPEG)
2. The app's "File Print" sends native format to its "CUPS client"
3. The Android CUPS client converts that native (eg JPEG) format to PDF
4. That CUPS client forwards that PDF to the printer at 192.168.1.20:????
5. The printer prints it because the printer accepts PDF format

Yes.

Hi Michael Logies,

Thanks for sticking with me as I generally don't give up easily, and even
when I find a solution (as I have, using Let's Print Droid), it's still
good for me to explore the CUPS solution, and then to write a tutorial for
others to do in minutes what has taken me (elapsed time) days to solve.

Thanks for confirming that the CUPS server is what converts the native
format to the printer format (which, in my HP LJ 2100tn, is PCL-5) via the
CUPS port (i.e., port 631, which I confirmed is open on the HP printer):
o https://i.postimg.cc/L4YqhR1P/printing16.jpg

I installed _every_ free app on both F-Droid & Google Play that had "CUPS"
in the name, and tested every single one; but all failed in my tests:
o https://play.google.com/store/search?q=cups%20printing&c=apps

Only belatedly did I realize there's just no way CUPS is going to work
unless I have a _separate_ dedicated Linux CUPS server running; although
there are heroic ways, if rooted, to run a Debian CUPS server on Android:
o How to print to CUPS (running on another computer in the LAN)?
https://android.stackexchange.com/questions/43774/how-to-print-to-cups-running-on-another-computer-in-the-lan

The chance of finding a robust CUPS server compiled for Windows is slim
o CUPS Server compiled for Windows?
https://stackoverflow.com/questions/2844819/cups-server-compiled-for-windows
"Making the complete CUPS server+filtering code work on Windows would be
a daunting task, since Windows spooling and printing is so much
different from the Unix world."
--
Solving problems nobody else has ever solved sometimes requires teamwork.