library(pdftools)
office_shot <- function( file, wd = getwd() ){
  cmd_ <- sprintf(
    "/Applications/LibreOffice.app/Contents/MacOS/soffice --headless --convert-to pdf --outdir %s %s",
    wd, file )
  system(cmd_)

  pdf_file <- gsub("\.(docx|pptx)$", ".pdf", basename(file))
  pdf_file
}
office_shot(file = "your_presentation.pptx")

library(RDCOMClient)

file <- "C:/path/to your/doc.docx"
wordApp <- COMCreate("Word.Application") #creates COM object
wordApp[["Documents"]]$Open(Filename=file) #opens your docx in wordApp
wordApp[["ActiveDocument"]]$SaveAs("C:/path/to your/doc.pdf"), FileFormat=17) #saves as PDF
wordApp$Quit() #quits the COM Word application