In order to attach a document to a webpage or email campaign, the document first needs to be uploaded to the system via the Documents module. For more information on how to do that, see the article How can I upload a new document?.
Once you upload your document, you need to get the URL to your document by clicking the actions button, clicking the Copy URL option in the context menu, and then clicking the "Copy to Clipboard" button in the revealed dialogue. You have now copied the URL to your document to your system's clipboard. Close the popup by clicking Close when done.
You can now attach this URL to your webpage or email campaign like attaching any other link. In order to do so, navigate to your webpage or email campaign and open the builder.
Now, select the portion of text you'd like to turn into a link, and click the Link button. Then, click once on the portion you've turned into a link, click the cogwheel icon in the right hand side panel to view Component Settings, and paste the URL in the Href field.
Once you publish the changes to your webpage or email campaign, you visitors or the recipients of your email will now be able to download the document you've attached by clicking the link.