I can send and receive files on the Portfolio via xterm2, on my modern PC I use TeraTerm to communicate with xterm2 over serial using the XModem protocol.
I also wrote an image converter to convert jpg to pgf (portfolio graphics file). I haven't yet implemented the compression to write pgc (portfolio graphics compressed) files directly. I use pgcompress on the atari to convert pgf to pgc.
Pgc files can be viewed on the Atari using pgshow.
The portfolio was my first Palmtop computer (of many -- the latest is a GPD Pocket 1). Unfortunately it died a long time ago. I miss the thing. It was fun to own.
An external keyboard would defeat the purpose! The benefit of the GPD Pocket is that you can throw it in your pocket and pull it out the compute. Admittedly the keyboard sucks -- the size and weird layout aren't the biggest problems; it's actually not that great at registering all keypresses. However, I can get used to it while coding. I think the Pocket 2 would be a lot better machine for coding.