hi, i want to use a 2003 powermac g5 as an ftp file server, how will it go?

Welcome!

The Lubuntu site (https://lubuntu.net/downloads/) still shows a download for PowerPC Macs so it sounds doable. However you may wish to look at their support page, especially regarding network protocol support.

The G5 tower has a 10/100/1000BASE-T ethernet port so speed should good. If it is a G5 iMac, they did not have 10/100/1000 until the May 2005 models. That could be a deal-breaker if you need speed.

It will work, but one thing you need to bear in mind is that these machine use a lot of power, comparable to a space heater. With one of these running 24/7 you're likely to see an increase in your utility costs.