convert .pptx and .xlsx files to pdf using libreoffice cli

I'm using LibreOffice CLI to convert two files I have:

• .XLSX to .PDF;
• .PPTX to .PDF;

To do the task I'm using the below command:

import subprocess
from subprocess import Popen, PIPE
import re

args = ['libreoffice', '--headless', '--convert-to',
        'pdf', '--outdir', 'output', 'my_file.pptx']
process = subprocess.run(args, stdout=subprocess.PIPE, stderr=subprocess.PIPE, timeout=25)
re.search('-> (.*?) using filter', process.stdout.decode())

It is converting the files to pdf, but unfortunately, it's not keeping the same layout style

Continuation of the previous page (with the footer) and header of the next page:

As you can see in the images, the headers have different size and it also has a weird value in the header "x000a"

I would like to understand if there are a alternative to fix this kind of problems?

Regards, Leonardo