For preserving both the existing DjVu OCR text layer and the TOC/bookmarks, use dpsprep. It is specifically built to transfer the DjVu hidden OCR text and document outline into the PDF.
sudo pacman -S djvulibre python-pipx
pipx install dpsprepThen go to the folder where the DjVu file is contained and open a terminal there.
dpsprep input.djvu output.pdf