Added type definition for function "pdf()" with buffer in callback.

types/nightmare/index.d.ts

@@ -107,6 +107,7 @@ declare class Nightmare {
     html(path: string, saveType: 'MHTML'): Nightmare;
     pdf(path: string): Nightmare;
     pdf(path: string, options: Object): Nightmare;
+    pdf(cd:(err:Error, data:Buffer)=>void): Nightmare;
     title(): string;
     title(cb: (title: string) => void): Nightmare;
     url(cb: (url: string) => void): Nightmare;

types/nightmare/nightmare-tests.ts

@@ -189,6 +189,14 @@ new Nightmare()
   .pdf('test/test.pdf')
   .run(done);
 
+new Nightmare()
+  .goto("http://yahoo.com")
+  .pdf((err,data)=>{
+    console.log(Buffer.isBuffer(data))
+  })
+  .run(done)
+
+
 new Nightmare()
   .goto('http://www.google.com/')
   .wait('input')