Shiny App not finding PDF file in www folder

Question

I am trying to create a simple button, link, and/or viewing pane in my Shiny App that allows users to download a pre-existing PDF file that I have saved in my www folder (code below). The App keeps coming back with the error message "File not Found". However, when I check that the file exists with file.exists("www/sample_label.pdf") it returns TRUE. I can't for the life of me figure out where the problem is coming from. The code works if I try a url like "https://cran.r-project.org/doc/manuals/r-release/R-intro.pdf". Any help would be greatly appreciated. Thanks

library(shiny)
ui <- fluidPage(
  titlePanel("Vaccine Label"),
  
    mainPanel(
    downloadButton("downloadLabel", label = "Download Label", class = NULL, icon = shiny::icon("download")),
    br(),
      tags$a("Download Label", href = "www/sample_label.pdf"),
    br(),
    tags$iframe(style="height:600px; width:100%; scrolling = yes", src = "www/sample_label.pdf")
    )
)

server <- function(input, output) {
  
 output$downloadLabel <-  downloadHandler(
    
    filename = "www/sample_label.pdf",
    content = function(file) {    
      file.copy("temp.pdf", file)
    }
  )
     
}

shinyApp(ui, server)