If you select a file from your Google Drive, you will need to sign into Google Drive first to allow PDF Mergy access to your Drive files. When you click Select Files from Google Drive, you will see a box on the screen with your name. Click your name.
You will see two options: 1) Connect itself to your Google Drive and 2) See, edit, create, and delete only the specific Google Drive files you use with this app. The first option (Connect itself to your Google Drive) gives PDF Mergy access to all of your Google Drive files including the files you do not use for PDF Mergy. The second option (See, edit, create, and delete only the specific Google Drive files you use with this app) means that PDF Mergy only has access to the files in Drive that you select to use for PDF Mergy. Check the box(es) depending on which option you want to use.
Note: If you do NOT want PDF Mergy to have access to your Google Drive, go into Drive and download those files directly to your computer. Then you can click the option Select Files from Your Computer when adding files to PDF Mergy.
Click Continue.
After selecting your first file, click the Select Files from Your Computer or Select Files from Google Drive button to add your second file to the list.
Once you have your files arranged in the order you want, click Merge.