fix #1802: change to = 'pdf' to to = 'latex', and update the non-working examples

Author: yihui

DESCRIPTION

@@ -1,7 +1,7 @@
 Package: rmarkdown
 Type: Package
 Title: Dynamic Documents for R
-Version: 2.1.8
+Version: 2.1.9
 Authors@R: c(
   person("JJ", "Allaire", role = "aut", email = "[email protected]"),
   person("Yihui", "Xie", role = c("aut", "cre"), email = "[email protected]", comment = c(ORCID = "0000-0003-0645-5666")),

NEWS.md

@@ -25,6 +25,8 @@ rmarkdown 2.2
 
 - `render()` now respects the YAML metadata in the R script when rendering the script with Pandoc 2.8 or later (thanks, @nsoranzo #1740, @cderv #1741).
 
+- For `pandoc_convert()`, when the argument `to = 'pdf'`, it will be changed to `'latex'` internally (thanks, @JohannesFriedrich, #1802).
+
 
 rmarkdown 2.1
 ================================================================================

R/pandoc.R

@@ -28,13 +28,13 @@
 #'
 #' # convert markdown to various formats
 #' pandoc_convert("input.md", to = "html")
-#' pandoc_convert("input.md", to = "pdf")
+#' pandoc_convert("input.md", to = "latex")
 #'
 #' # process citations
 #' pandoc_convert("input.md", to = "html", citeproc = TRUE)
 #'
 #' # add some pandoc options
-#' pandoc_convert("input.md", to="pdf", options = c("--listings"))
+#' pandoc_convert("input.md", to = "latex", options = c("--listings"))
 #' }
 #' @export
 pandoc_convert <- function(input,
@@ -61,6 +61,7 @@ pandoc_convert <- function(input,
   args <- c(input)
   if (!is.null(to)) {
     if (to == 'html') to <- 'html4'
+    if (to == 'pdf') to <- 'latex'
     args <- c(args, "--to", to)
   }
   if (!is.null(from))