After you have uploaded your document to the Library you can now Insert a link to that document within your campaigns. When editing an email you will want to edit the block, highlight the text you want as the link (optional), then from the Insert Menu on the left you will choose Insert > Document Link. You will then see a window with all of the documents uploaded to your account. If you did not highlight text for the link you will need to type text at the top of the window in the field for Link Text. With the icons next to this field you can also style the link if you would like (you can change the Color of the link, bold, italicize or underline the text). You will then select the document you wish to link to within the list and hit Insert.
In Order to test out the Link you can Save the block you are working in and Preview the Campaign. From the Preview window you can click on the link and it should open up your document for you.
You can also add the Document link to your landing page or make it the link behind an image. To do this you will want to go to the Library tab down the left hand side of the screen. You can then find the document that you wish to link to. If you click on the Preview of the email or the name of the document it will open a new window with specifics of that document. You will see below the icon of the file type (i.e. PDF, DOC) there is a File URL field. You can click select to highlight the link and Copy it. You will then go to the Campaign and choose to Insert a new link, or customize an Image to add a link to it.