A4横のファイルを上下に並べて A3 にします。

プログラム

pdf_merge_landscape.py

#! /usr/bin/python
#
#	pdf_merge_landscape.py
#
#					   Feb/01/2023
#
import  sys
import  PyPDF2
from PyPDF2 import Transformation
#
# ------------------------------------------------------------------
def show_size_proc(page_obj_in):
	print(page_obj_in.artbox.lower_left)
	print(page_obj_in.artbox.lower_right)
	print(page_obj_in.artbox.upper_left)
	print(page_obj_in.artbox.upper_right)
# ------------------------------------------------------------------
def pdf_page_read_proc(pdf_file_in):
	pdf_file = open(pdf_file_in, 'rb')
	pdf_reader = PyPDF2.PdfReader(pdf_file)
	page_obj = pdf_reader.pages[0]
#
	show_size_proc(page_obj)
#
	return	pdf_file,page_obj
# ------------------------------------------------------------------
def pdf_merge_proc(page_obj_a,page_obj_b):
	deltx = 960
	delty = 1080
	page_obj_out = PyPDF2.PageObject.create_blank_page(width=deltx,height=delty)
	page_obj_out_bb = PyPDF2.PageObject.create_blank_page(width=deltx,height=delty)

	op = Transformation().translate(tx=0, ty=540)
	page_obj_out.merge_page(page_obj_a)
	page_obj_out.add_transformation(op)

	page_obj_out_bb.merge_page(page_obj_b)
	page_obj_out.merge_page(page_obj_out_bb) 
#
	show_size_proc(page_obj_out)
#
	pdf_writer = PyPDF2.PdfWriter()
	pdf_writer.add_page(page_obj_out)
#
	return pdf_writer
# ------------------------------------------------------------------
sys.stderr.write ("*** 開始 ***\n")
sys.stderr.write("PyPDF2.__version__ = %s\n" % PyPDF2.__version__)
pdf_in_a = sys.argv[1]
pdf_in_b = sys.argv[2]
pdf_out = sys.argv[3]
#
pdf_file1,page_obj_a = pdf_page_read_proc(pdf_in_a)
pdf_file2,page_obj_b = pdf_page_read_proc(pdf_in_b)
#
pdf_writer = pdf_merge_proc(page_obj_a,page_obj_b)
#
pdf_file1.close()
pdf_file2.close()
#
pdf_output_file = open(pdf_out, 'wb')
pdf_writer.write(pdf_output_file)
#
pdf_output_file.close()
#
sys.stderr.write ("*** 終了 ***\n")
# ------------------------------------------------------------------

実行コマンド
入力 in01.pdf (A4) in02.pdf (A4)
出力　out01.pdf (A3)

./pdf_merge_landscape.py in01.pdf in02.pdf out01.pdf

実行結果

$ ./pdf_merge_landscape.py in01.pdf in02.pdf out01.pdf
*** 開始 ***
PyPDF2.__version__ = 3.0.1
(0, 0)
(960, 0)
(0, 540)
(960, 540)
(0, 0)
(960, 0)
(0, 540)
(960, 540)
(0, 0)
(960, 0)
(0, 1080)
(960, 1080)
*** 終了 ***

次のバージョンで確認しました。

$ python --version
Python 3.10.7

>>> import  PyPDF2
>>> PyPDF2.__version__
'3.0.1'