When you need to sign a PDF document, it's quite easy to do in Libreoffice Draw for instance. You can just insert an image of your signature and print to PDF afterwards. Sometimes LibreOffice will not do a great job with opening some types of PDF documents. In these cases you can try to use "Lumin PDF" which can be accessed directly from Google Drive when you open a PDF document. You will be asked to accept the Lumin PDF connection to your Google account. You will need to accept the conditions and you are ready to go.
