Shiny PDF output problem

server.R

require(shiny)
library(ggplot2)

shinyServer(function(input, output) {
  datasetInput <- reactive({
    switch(input$dataset,
           "rock" = rock,
           "pressure" = pressure,
           "cars" = cars)
  })

  plotInput <- reactive({
    df <- datasetInput()
    plot(df[[1]], df[[2]])
  })

  output$plot <- renderPlot({
    print(plotInput())
  })

output$downloadPlot <- downloadHandler(
    filename = function() { paste(input$dataset, '.pdf', sep='') },
    content = function(file) {
      pdf(file, width=8, height=6)
      plotInput()
      dev.off()
    })
})

ui.R

shinyUI(pageWithSidebar(
  headerPanel('Downloading Data'),
  sidebarPanel(
selectInput("dataset", "Choose a dataset:", 
            choices = c("rock", "pressure", "cars")),
    downloadButton('downloadPlot', 'Download Plot')
  ),
  mainPanel(
    plotOutput('plot')
  )
))