Output (non-ggplot) Shiny plots to PDF

server.R

library(shiny)
shinyServer(
  function(input, output) {
    
    plotInput <- reactive({
      if(input$returnpdf){
        pdf("plot.pdf", width=as.numeric(input$w), height=as.numeric(input$h))
        plot(rnorm(sample(100:1000,1)))
        dev.off()
      }
      plot(rnorm(sample(100:1000,1)))
    })

    output$myplot <- renderPlot({ plotInput() })
    output$pdflink <- downloadHandler(
      filename <- "myplot.pdf",
      content <- function(file) {
        file.copy("plot.pdf", file)
      }
    )
  }
)

ui.R

require(shiny)
pageWithSidebar(
  headerPanel("Output to PDF"),
  sidebarPanel(
    checkboxInput('returnpdf', 'output pdf?', FALSE),
    conditionalPanel(
      condition = "input.returnpdf == true",
      strong("PDF size (inches):"),
      sliderInput(inputId="w", label = "width:", min=3, max=20, value=8, width=100, ticks=F),
      sliderInput(inputId="h", label = "height:", min=3, max=20, value=6, width=100, ticks=F),
      br(),
      downloadLink('pdflink')
    )
  ),
  mainPanel({ mainPanel(plotOutput("myplot")) })
)