Rshiny输出pdf损坏问题

尝试利用RShiny在网页中设置Download按钮，下载ggplot2绘制的图（存储在`graph`变量中），但存储到本地后打开pdf，出现“文件已经损坏”。代码如下

## Only run examples in interactive R sessions
if (interactive()) {
  
  ui <- fluidPage(
    downloadLink(outputId = "downloadGraph", 
                 label = "Download")
  )
  
  server <- function(input, output) {
    # Our dataset
    data <- mtcars
    
    output$downloadGraph <- downloadHandler(
      filename = function() {
        paste("graph-", Sys.Date(), ".pdf", sep="")
      },
      content = function(file) {
        pdf(file)
        graph  # ggplot2绘图存储的变量
        dev.off()
      }
    )
  }
  
  shinyApp(ui, server)
}