diff --git a/README.md b/README.md
index 00511a0..7c454c5 100644
--- a/README.md
+++ b/README.md
@@ -11,7 +11,7 @@ Vapor 4 library for converting HTML (Leaf or otherwise) into PDF files using
 
 Add the following in your `Package.swift` file
 ```Swift
-.package(url: "https://github.com/vapor-community/wkhtmltopdf.git", from: "3.0.0"),
+.package(url: "https://github.com/vapor-community/wkhtmltopdf.git", from: "4.0.0"),
 ```
 
 ## 📘 Overview
@@ -22,37 +22,39 @@ in the `Document` initialiser. The default is `/usr/local/bin/wkhtmltopdf`.
 Run it to ensure it and any dependencies are installed correctly.
 
 To create a PDF, create and configure a `Document`, add one or more `Page`s,
-and then call `generatePDF(on: Request)`. Here is a full example:
+and then call `generatePDF(on: threadPool, eventLoop: eventLoop)`. Here is a full example using Vapor:
 
 ```Swift
 import wkhtmltopdf
 
-func pdf(_ req: Request) -> Future<Response> {
-     // Create document. Margins in mm, can be set individually or all at once.
+func pdf(_ req: Request) -> EventLoopFuture<Response> {
+    // Create document. Margins in mm, can be set individually or all at once.
     // If no margins are set, the default is 20mm.
     let document = Document(margins: 15)
     // Create a page from an HTML string.
     let page1 = Page("<p>Page from direct HTML</p>")
 
     // Create a page from a Leaf template.
-    let page2 = try req.view().render("page_from_leaf_template")
+    let page2 = req.view.render("page_from_leaf_template")
 
     // Create a page from a Leaf template with Context variables.
-    let page3 = try req.view().render("page_from_leaf_template", [ "firstName": "Peter",
-                                                               "lastName": "Pan"])
-    let pages = [ page2, page3].flatten(on: req)
+    let context = ["firstName": "Peter", "lastName": "Pan"]
+    let page3 = req.view.render("page_from_leaf_template", context)
+
+    let pages = [ page2, page3]
+        .flatten(on: req.eventLoop)
         .map { views in
-            return views.map { Page($0.data) }
+            views.map { Page($0.data) }
         }
 
     return pages.flatMap { pages in
         // Add the pages to the document
         document.pages = [page1] + pages
         // Render to a PDF
-        let pdf = try document.generatePDF(eventLoop: req.eventLoop)
+        let pdf = document.generatePDF(on: req.application.threadPool, eventLoop: req.eventLoop)
         // Now you can return the PDF as a response, if you want
-        return pdf.map { data -> Response in
-            return HTTPResponse(
+        return pdf.map { data in
+            return Response(
                 status: .ok,
                 headers: HTTPHeaders([("Content-Type", "application/pdf")]),
                 body: .init(data: data)
diff --git a/Sources/wkhtmltopdf/Document+Generate.swift b/Sources/wkhtmltopdf/Document+Generate.swift
index cbbae26..4ad2c62 100644
--- a/Sources/wkhtmltopdf/Document+Generate.swift
+++ b/Sources/wkhtmltopdf/Document+Generate.swift
@@ -3,8 +3,7 @@ import NIO
 
 extension Document {
 
-    public func generatePDF(on threadPool: NIOThreadPool = NIOThreadPool(numberOfThreads: 1), eventLoop: EventLoop) throws -> EventLoopFuture<Data> {
-        threadPool.start()
+    public func generatePDF(on threadPool: NIOThreadPool, eventLoop: EventLoop) -> EventLoopFuture<Data> {
         return threadPool.runIfActive(eventLoop: eventLoop) {
             let fileManager = FileManager.default
 
diff --git a/Sources/wkhtmltopdf/Document.swift b/Sources/wkhtmltopdf/Document.swift
index fdd0541..7b3c832 100644
--- a/Sources/wkhtmltopdf/Document.swift
+++ b/Sources/wkhtmltopdf/Document.swift
@@ -16,17 +16,26 @@ public class Document {
     let wkArgs: [String]
 
     public var pages: [Page] = []
-    
-    public init(size: String = "A4", zoom: String? = nil, margins all: Int? = nil, top: Int? = nil, right: Int? = nil, bottom: Int? = nil, left: Int? = nil, path: String = "/usr/local/bin/wkhtmltopdf", wkArgs: [String]? = nil) {
-        self.zoom = zoom ?? "1.3"
+
+    public init(size: String = "A4", zoom: String = "1.3", margins: Int = 20, path: String = "/usr/local/bin/wkhtmltopdf", wkArgs: [String] = []) {
+        self.zoom = zoom
         self.paperSize = size
-        self.topMargin = all ?? top ?? 20
-        self.rightMargin = all ?? right ?? 20
-        self.bottomMargin = all ?? bottom ?? 20
-        self.leftMargin = all ?? left ?? 20
+        self.topMargin = margins
+        self.rightMargin = margins
+        self.bottomMargin = margins
+        self.leftMargin = margins
         self.launchPath = path
-        
-        self.wkArgs = wkArgs ?? []
+        self.wkArgs = wkArgs
     }
 
+    public init(size: String = "A4", zoom: String = "1.3", top: Int = 20, right: Int = 20, bottom: Int = 20, left: Int = 20, path: String = "/usr/local/bin/wkhtmltopdf", wkArgs: [String] = []) {
+        self.zoom = zoom
+        self.paperSize = size
+        self.topMargin = top
+        self.rightMargin = right
+        self.bottomMargin = bottom
+        self.leftMargin = left
+        self.launchPath = path
+        self.wkArgs = wkArgs
+    }
 }
diff --git a/Sources/wkhtmltopdf/Page.swift b/Sources/wkhtmltopdf/Page.swift
index d05d38c..3d97e80 100644
--- a/Sources/wkhtmltopdf/Page.swift
+++ b/Sources/wkhtmltopdf/Page.swift
@@ -1,4 +1,5 @@
 import Foundation
+import NIO
 
 public struct Page {
     let content: Data
@@ -7,6 +8,10 @@ public struct Page {
         self.content = content
     }
 
+    public init(_ content: ByteBuffer) {
+        self.content = Data(content.readableBytesView)
+    }
+
     public init(_ content: String) {
         self.content = Data(content.utf8)
     }
diff --git a/Tests/wkhtmltopdfTests/wkhtmltopdfTests.swift b/Tests/wkhtmltopdfTests/wkhtmltopdfTests.swift
index 261e9f6..97744cc 100644
--- a/Tests/wkhtmltopdfTests/wkhtmltopdfTests.swift
+++ b/Tests/wkhtmltopdfTests/wkhtmltopdfTests.swift
@@ -24,7 +24,10 @@ class wkhtmltopdfTests: XCTestCase {
         let document = Document(margins: 15)
         let page1 = Page("<p>Page from direct HTML</p>")
         document.pages = [page1]
-        let data = try document.generatePDF(eventLoop: eventLoop).wait()
+        let threadPool = NIOThreadPool(numberOfThreads: 1)
+        threadPool.start()
+        let data = try document.generatePDF(on: threadPool, eventLoop: eventLoop).wait()
+        try threadPool.syncShutdownGracefully()
         // Cop-out test, just ensuring that the returned data is something
         XCTAssert(data.count > 50)
         // Visual test