JoinPDFs.scala

val mainSq = (userHome / "Downloads" / "rb").children(f => f.extL == "pdf" && f.base.startsWith("p-")).sorted(File.NameOrdering)
mainSq.foreach(f => println(f.base))

def runMain(): Unit = {
  val out = userHome / "Downloads" / "rb" / "main.pdf"
  require (!out.exists())
  import scala.sys.process._
  val cmd = Seq("pdftk") ++ mainSq.map(_.path) ++ Seq("cat", "output", out.path)
  val code: Int = cmd.!
  require (code == 0)
}

runMain()