Do I have to download AirPrint printer on my iphone

Restarting your printer and phone as troubleshooting steps are generally not recommended as useful troubleshooting steps. You're best off following the AirPrint troubleshooting guide: https://support.apple.com/en-ca/HT201387

For the AirPrint feature of your iPhone to work, your printer must support AirPrint and have it enabled. If it worked before, but it doesn't work now, then perhaps there is a known issue with your printer or your wi-fi. What model printer do you have?

First, see if another Mac or PC connected to your wi-fi can print to your printer. If your computer can see the printer but your iPhone cannot, then it's time to check your wi-fi network.

Many printers offer a web interface, such as http://192.168.0.[somenumber]. If you can determine what your printer's web interface address is, see if Safari on your iPhone can reach that address. If Safari on your iPhone can see the printer's web interface, but AirPrint cannot see the printer, then the problem is AirPrint support on your printer. If Safari on your iPhone cannot see the printer's web interface, then there is a wi-fi problem.

I know from personal experience that wi-fi is usually the culprit. For example, if your wi-fi has a split 5ghz for 802.11ac (preferred by iPhone 6s) and 2.4ghz for 802.11n (generally used by printers) then your phone and printer may or may not see each other, depending on how your wi-fi treats these two methods of connecting: as one subnet, or separate subnets. Verify that your printer and iPhone are connected to the same wi-fi network. Then, verify that AirPrint is enabled on your printer. Your printer should appear in your iPhone in a moment.

No, it should be on your iPhone as a feature (it's not an app).

If your iPhone cannot "see" your printer (with Airprint capabilities, listed here:--> About AirPrint - Apple Support), try this workaround:

• Turn off your printer & your iPhone.
• Restart your modem-router, waiting for its complete autoconfiguration
• Turn your printer back on, and check if it is correctly connected to your wifi network
• Turn on your iPhone, connect it to your wifi network, and test the connection with that printer again

Regards

Giulio

See also this Apple article: Use AirPrint to print from your iPhone, iPad, or iPod touch

https://support.apple.com/en-ca/HT201387