Logos can be applied in two ways: 

You can either embed any logos and signatures you require in your individual document, or you can use a ‘Background’. 

Backgrounds are templates set up and held in your Library which can be applied to documents as required. 

You could, for example, have a PDF background of your company logo and use this with each document requiring the logo. 

See the article ‘Backgrounds’ under the Library for further information.