Can I use iPad's charger for iPhone?

Yes, it is quite fine to use the iPad charger with other iDevices.

when it comes to chargers

there are 2 things

1 volt if it's much higher then it will damage the device if it's much lower the device will not detect the charger

2 amps if the charger can't support enough amps vs. what a device try to draw it will not charge as ohms law result in the amps not being provided will automaticly lower the volts (see 1)

if apple have a support documents in reguard to ohms law I can't say

CSarath wrote:

Is there any support documents available at apple website in this regard?

iPhone, iPod, Using the iPad Charger

http://support.apple.com/kb/HT4327