How to make this Python script to run subfolders too?

Which part of the codes do I need to change in order to include subfolders?

File handle.py

import glob
import os
import sys
from typing import List


def get_filenames(filepath: str, pattern: str) -> List[str]:
    """Returns all filenames that matches the pattern in current folder.

    Args:
        filepath (str): folder path.
        pattern (str): filename pattern.

    Returns:
        List[str]: list of paths.
    """
    filenames = glob.glob(os.path.join(filepath, pattern))
    if filenames:
        return filenames
    return sys.exit("Error: no file found, check the documentation for more info.")

Main.py

import math

import click

import pdf_split_tool.file_handler
import pdf_split_tool.pdf_splitter


def _confirm_split_file(filepath: str, max_size_bytes: int) -> None:
    """Split file if user confirms or is valid.

    Args:
        filepath: PDF path.
        max_size_bytes: max size in bytes.
    """
    splitter = pdf_split_tool.pdf_splitter.PdfSplitter(filepath)
    valid = True
    if not valid:
        click.secho(
            (
                "Warning: {} has more than 200kb per page. "
                "Consider reducing resolution before splitting."
            ).format(filepath),
            fg="yellow",
        )
        if not click.confirm("Do you want to continue?"):
            click.secho("{} skipped.".format(filepath), fg="blue")
            return
    splitter.split_max_size(max_size_bytes)


@click.command()
@click.version_option()
@click.argument("filepath", type=click.Path(exists=True), default=".")
@click.option(
    "-m",
    "--max-size",
    type=float,
    help="Max size in megabytes.",
    default=20,
    show_default=True,
)
def main(filepath: str, max_size: float) -> None:
    """Pdf Split Tool."""
    max_size_bytes = math.floor(max_size * 1024 * 1024)  # convert to bytes
    if filepath.endswith(".pdf"):
        _confirm_split_file(filepath, max_size_bytes)
    else:
        filepaths = pdf_split_tool.file_handler.get_filenames(filepath, "*.pdf")
        for path in filepaths:
            _confirm_split_file(path, max_size_bytes)


if __name__ == "__main__":
    main(prog_name="pdf-split-tool")  # pragma: no cover

pdf_splitter.py

import os
import sys
import tempfile

import PyPDF4


class PdfSplitter:
    """Pdf Splitter class."""

    def __init__(self, filepath: str) -> None:
        """Constructor."""
        self.filepath = filepath
        self.input_pdf = PyPDF4.PdfFileReader(filepath, "rb")
        self.total_pages = self.input_pdf.getNumPages()
        self.size = os.path.getsize(filepath)
        self.avg_size = self.size / self.total_pages
        print(
            "File: {}\nFile size: {}\nTotal pages: {}\nAverage size: {}".format(
                filepath, self.size, self.total_pages, self.avg_size
            )
        )


    def _get_pdf_size(self, pdf_writer: PyPDF4.PdfFileWriter) -> int:
        """Generates temporary PDF.

        Args:
            pdf_writer: pdf writer.

        Returns:
            int: generated file size.
        """
        with tempfile.TemporaryFile(mode="wb") as fp:
            pdf_writer.write(fp)
            return fp.tell()

    def split_max_size(self, max_size: int) -> int:
        """Creates new files based on max size.

        Args:
            max_size: size in integer megabytes.

        Returns:
            int: number of PDFs created.
        """
        if self.size > max_size:
            avg_step = int(max_size / self.avg_size)

            pdfs_count = 0
            current_page = 0

            while current_page != self.total_pages:
                end_page = current_page + avg_step
                if end_page > self.total_pages:
                    end_page = self.total_pages

                current_size = sys.maxsize

                # while PDF is too big create smaller PDFs
                while current_size > max_size:
                    pdf_writer = PyPDF4.PdfFileWriter()
                    for page in range(current_page, end_page):
                        pdf_writer.addPage(self.input_pdf.getPage(page))
                    current_size = self._get_pdf_size(pdf_writer)
                    self.input_pdf = PyPDF4.PdfFileReader(self.filepath, "rb")
                    end_page -= 1

                # write PDF with size max_size
                with open(
                    self.filepath.replace(".pdf", "-{}.pdf".format(pdfs_count)), "wb"
                ) as out:
                    pdf_writer.write(out)

                current_page = end_page + 1
                pdfs_count += 1
            return pdfs_count
        return 0