Return the total number of pages in a PDF file.

pdf.file.page.number <- function(fname) {
    ## Return the total number of pages in a PDF file.
    ## Require Linux program "pdfinfo"
    a <- pipe(paste0("pdfinfo '", fname, "' | grep Pages | cut -d: -f2"))
    page.number <- as.numeric(readLines(a))
    close(a)
    page.number
}
if (F) {
    ## pdf.file.page.number("ALL-Plots.pdf")
}