To add this to your list detail page, simply add a new or edit an existing list item. At the very bottom of that page, in the Links section, are two areas to select a document and upload new documents. Documents are shared between custom pages and lists so any uploaded in either place will show as optional items here.
You can give the button a name or use the default name based on type. Word docs will default to 'View Document', Powerpoints will default to 'View Powerpoint', and PDFs will default to 'View PDF'.
You can upload Word Documents (.doc and .docx files), Powerpoint Files (.ppt and .pptx files), and PDF documents (.pdf files). You may add up to two per list item but files are limited to 4MB in size each.
These documents can also be added to custom pages. Learn more here.
Lists