How open in new Tab for PageReference?

To open the URL in new Tab for PageReference, kindly use <apex:commandLink/>.

Sample Code:

Visualforce page:

<apex:page controller="Sample">
<apex:form >
    <apex:pageblock >
        <apex:commandlink action="{!switch}" target="_blank">
            <apex:commandButton value="Switch"/>
        </apex:commandLink>
    </apex:pageblock>
</apex:form>
</apex:page>

Apex Controller:

public class Sample {  
   
    public Sample() {
    }
   
    public pageReference switch() {
        PageReference pageRef = new PageReference('http://www.google.com');
        pageRef.setRedirect(true);
        return pageRef;
    }           
}