Reoder pdf with drag and drop in shiny

app.R

# https://twitter.com/henrikbengtsson/status/1583520708470067201

library(shiny)
library(sortable)
library(pdftools)
library(magick)

# The file to manipulate
pdffile <- tempfile(fileext = '.pdf')
download.file('https://www.uscis.gov/sites/default/files/document/forms/i-9-paper-version.pdf', pdffile)

# convert the pdf into thumbnails
thumbs <- magick::image_read_pdf(pdffile, density = 200, pages = 1:3) |> 
  image_resize("x100") |> 
  lapply(function(img){
    tmp <- magick::image_write(img, tempfile(fileext = '.png'), format = 'png')
    thumbnail <- tags$img(src = file.path('thumbs', basename(tmp)), height = 100)
    tags$figure(align = "center", thumbnail, alt = tmp)
  })
names(thumbs) = seq_along(thumbs)

addResourcePath(prefix = "thumbs", directoryPath = tempdir())

rank_list_basic <- rank_list(
  text = "Drag the items in any desired order",
  labels = thumbs,
  input_id = "rank_list_basic"
)

ui <- fluidPage(
  fluidRow(
    column(
      width = 12,
      tags$h2("PDF page reorder"),
      tags$b("Exercise"),
      rank_list_basic,
      tags$b("Order"),
      verbatimTextOutput("results_basic"),
      tags$a(href = 'thumbs/output.pdf', 'Download PDF', target = '_blank')
    )
  )
)

server <- function(input, output, session) {
  output$results_basic <- renderPrint({
    pages <- as.numeric(input$rank_list_basic)
    if(!length(pages)){
      pages <- seq_along(thumbs)
    }
    qpdf::pdf_subset(pdffile, pages = pages, output = file.path(tempdir(), 'output.pdf'))
    pages
  })
}

shinyApp(ui, server)