As already mentioned, the theme you pick will customize your PDF.
Another option is using Pandoc, but it takes some technical knowledge. It allows you to completely customize the PDF generated from a markdown file, but it won’t be 100% compatible with Obsidian Markdown.
Take a look at Zettlr, it processes markdown files before running them through pandoc and you can use custom latex templates. Some folks use Obsidian for their actual PKM then Zettlr for exporting PDFs since it’s easier than using Pandoc from the command line.