I am trying to cover this class but unable to cover `url = '/servlet/servlet.FileDownload?file='+lstDocument[0].id;` and pagereference method. Need help in increasing test coverage.

Class:

    public class documentctr {
    
    public String docName{get;set;}
    public String url{get;set;}
    
    public documentctr(){
        
        if(ApexPages.CurrentPage().getParameters().get('doc') != null){
        
            docName =  ApexPages.CurrentPage().getParameters().get('doc');
            List<Document> lstDocument = [select id from Document where DeveloperName =: docName];    
            if(lstDocument.size() > 0){
                url = '/servlet/servlet.FileDownload?file='+lstDocument[0].id;
            }
        }
    }
    
    public pagereference returnUrl(){
        
        if(url != null){
            pagereference page = new pagereference(url);
            page.setRedirect(true);
            return page;
        }
        return null;
    }
}
    
    
Test Class:
    
    @isTest

    public class documentctrTracker{
    
    static Folder objFolder;
    public String url;
    
    static testMethod void firstMethod() {
        
        loadData();
        
              
        documentctr ob1 = new documentctr();
        PageReference pageRef = ob1 .returnUrl();
        //ob1.returnUrl();
        
        PageReference newPage = Page.pvdocload;
        Test.setCurrentPage(newPage);
        newPage.getParameters().put('doc','newFile');
        //ApexPages.currentPage().getParameters().put('doc','newFile');
        documentctr ob = new documentctr();
        ob.returnUrl();
    }
    
    static void loadData(){
    
        insertFolder();
        insertDocument();
    }
    
    static void insertFolder(){
        
        objFolder = [select id from Folder where Type = 'Document' limit 1];        
    }
    
    static void insertDocument(){  
        
        Document obj = new Document();
        obj.Name = 'newFile';
        obj.FolderId = objFolder.Id;
        insert obj;
         
       
       // ApexPages.currentPage().getParameters().put('doc','newFile');
        ApexPages.Standardcontroller sc = new ApexPages.Standardcontroller(obj);
        documentctr ob1 = new documentctr();
        
        try{
        PageReference newPage = Page.pvdocload;
        Test.setCurrentPage(newPage);
        newPage.getParameters().put('doc','newFile');
        ob1.returnUrl();
        }
        catch(DmlException e){
        }
    }