Aras Innovator manages data using a number of different "Item Types". Items can be things like Parts, Projects, Process Plans, etc. The type of item used by Aras to manage files is called a "Document". An Aras Document should be thought of as a container that can hold one or more files and has properties (metadata) describing the content of the files. Aras Documents can be linked to other item types like Parts and Projects using relationships, so that when someone looks at a Part for example, they can easily navigate to the related Document to find and view the files.
While Aras Documents can technically contain any arbitrary number of files, when you use the Office Connector, you will have only 1 or 2 files per Document. One file will be the native Office file (like .docx or .pptx), and the other will be a viewable file (like .pdf), which is optional. The one exception to this rule is that when saving Outlook e-mails to Aras, there will be one file for the message body, and any number of other files which are the attachments from the e-mail.