If you don't have many files, the best option is to convert them to template literals, and to .js files, something like so:
Initial:
example.md
# markdown
something else
Final:
example.js
const example = `# markdown
something else
`;
export default example;
and then, you would be able to use, for example, react-native-simple-markdown to render it like so:
<SimpleMarkdown>
{example}
</SimpleMarkdown>
Even if there are many files, if they will already be bundled with the app, you can set up a script, or maybe a gulp task, to convert them before building the app.
However, if the user will be uploading these files, or you don't like this solution, there might be another way, either accessing the file system or using a local website inside the app.