Use the default download button

import { Viewer } from '@react-pdf-viewer/core';
import { getFilePlugin } from '@react-pdf-viewer/get-file';

// Your render function
const getFilePluginInstance = getFilePlugin();
const { DownloadButton } = getFilePluginInstance;

return (
    <div
        className="rpv-core__viewer"
        style={{
            border: '1px solid rgba(0, 0, 0, 0.3)',
            display: 'flex',
            flexDirection: 'column',
            height: '100%',
        }}
    >
        <div
            style={{
                alignItems: 'center',
                backgroundColor: '#eeeeee',
                borderBottom: '1px solid rgba(0, 0, 0, 0.3)',
                display: 'flex',
                padding: '4px',
            }}
        >
            <DownloadButton />
        </div>
        <div
            style={{
                flex: 1,
                overflow: 'hidden',
            }}
        >
            <Viewer fileUrl="/assets/pdf-open-parameters.pdf" plugins={[getFilePluginInstance]} />
        </div>
    </div>
);