@GreyMerchant One option to rotate a PDF is to use the Python package PyPDF2. The code chunk below rotates the manual An Introduction to R by 180 degrees:
import PyPDF2
original = open('R-intro.pdf', 'rb')
pdf_in = PyPDF2.PdfFileReader(original)
rotated = open('R-intro-upside-down.pdf', 'wb')
pdf_out = PyPDF2.PdfFileWriter()
for pg in range(pdf_in.numPages):
page = pdf_in.getPage(pg)
page.rotateClockwise(180)
pdf_out.addPage(page)
pdf_out.write(rotated)
rotated.close()
original.close()
Depending on your use case, you could run these lines manually, convert it to a command-line Python script, or integrate it into an R script via reticulate.