I have a powerbook g4 and it runs panther os x, can I upgrade the os digitally to leopard for free?

No you need to purchase the install DVDs (retail copy) from Apple. If no longer available you may find a set on eBay or Amazon.

Cheers

Pete

No. Apple doesn't sell Leopard any longer, so you will need to purchase a retail copy of Leopard on eBay or Amazon.com. Please be sure your model is able to run Leopard. You must have a 1 GHz or 867 MHz CPU or better in order to upgrade. If that be the case then you can upgrade to Tiger - 10.4.11 or earlier.

Leopard requirements/10.5.x...

* Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor

minimum system requirements

* 512MB of memory (I say 1.5GB for PPC at least, 2-3GB minimum for IntelMacs)

* DVD drive for installation

* 9GB of available disk space (I say 30GB at least)

Classic/OS9 Apps no longer supported.

http://bobskelley.com/2015/01/04/9-reasons-leopard-keeps-power-macs-real-in-2015 /

Trouble is Apple no longer sells it, check eBay & such for the Retail version, not the Gray Discs...

http://www.ebay.com/sch/i.html?_nkw=mac+os+x+leopard+retail+10.5

There are workarounds if the 867MHz CPU is the only hangup...

http://sourceforge.net/projects/leopardassist/

http://lowendmac.com/osx/leopard/unsupported.html

List of Applications Not Compatible with Leopard...

http://guides.macrumors.com/List:Applications_Not_Compatible_with_Leopard