This is an example function to pick document from the device, attach it with the email and open in the email application(gmail etc.)

Mailer.js

/*
  Call pickDocAndSendToMail function on button click
*/

//...other imports

import DocumentPicker from 'react-native-document-picker';
import Mailer from 'react-native-mail';
import RNFS from 'react-native-fs';

pickDocAndSendToMail = async () => {
        try {
            const res = await DocumentPicker.pick({
                type: [DocumentPicker.types.pdf],
            });
            console.log('document picker response is: ', res);

            let path = res.uri;
            if (Platform.OS !== 'ios') {
                try {
                    let hasPermission = await PermissionsAndroid.check(PermissionsAndroid.PERMISSIONS.WRITE_EXTERNAL_STORAGE);
                    if (!hasPermission) {
                        const granted = await PermissionsAndroid.request(
                            PermissionsAndroid.PERMISSIONS.WRITE_EXTERNAL_STORAGE
                        );
                        hasPermission = granted !== PermissionsAndroid.RESULTS.GRANTED;
                    }
                    if (!hasPermission) {
                        console.log('permission denied');
                        return;
                    }
                } catch (error) {
                    console.warn(error);
                }

                path = `${RNFS.ExternalStorageDirectoryPath}/project_overview_${Number(new Date())}.pdf`;

                try {
                    await RNFS.copyFile(res.uri, path);
                } catch (error) {
                    console.log('file copy error: ', error);
                    return;
                }
            }

            Mailer.mail({
                subject: 'need help',
                recipients: [],
                body: '<b>A Bold Body</b>',
                isHTML: true,
                attachment: {
                    path, // The absolute path of the file from which to read data.
                    type: 'pdf', // Mime Type: jpg, png, doc, ppt, html, pdf, csv
                    name: 'project_overview.pdf', // Optional: Custom filename for attachment
                }
            }, (error, event) => {
                if (error) {
                    console.log('mailer error: ', error);
                }
            });
        } catch (err) {
            if (DocumentPicker.isCancel(err)) {
                // User cancelled the picker, exit any dialogs or menus and move on
            } else {
                throw err;
            }
        }
    };

//...other code