To upload a file into Eloqua:
Go to: Assets>Components>File Storage>WSB>(Your Department)
Click "Upload" in the top right corner, and browse for the file you want to upload. Once uploaded, you can double click the file and you can see the generated URL, which you can link from anywhere.
To link to the file from an Eloqua email:
From within the email builder, highlight the text you want to link and click the Hyperlink button in the toolbar (same as how you link to a website). From the left sidebar, select "File in file storage" from the Link Type dropdown, then browse for the file you uploaded.