from pypdf import PdfReader, PdfWriter

reader = PdfReader("pages_11_20.pdf")
writer1 = PdfWriter()
for i in range(0, 5):
    writer1.add_page(reader.pages[i])
with open("part1.pdf", "wb") as f:
    writer1.write(f)

writer2 = PdfWriter()
for i in range(5, 10):
    writer2.add_page(reader.pages[i])
with open("part2.pdf", "wb") as f:
    writer2.write(f)