All the messenger logs will be stored under
.purpledirectory located in your home directory.
this is the location where all your yahoo messenger chat logs will be stored.
You can also view the logs by selecting "Conversation -> View Log" when you have the contact window opened.
To send a message to a offline contact, use "Offline Message Emulation". Open "Tool->Plugins" and make sure you have selected "Offline Message Emulation". This will save the message you sent to a offline contact and the message will be delivered once he is online.
If you are offline and a contact sends you a message that will be delivered to you when you come online.
There are other good plugins for pidgin. To install the extra plugins use the command
sudo apt-get install pidgin-plugin-pack