Documents

When to use: avoid and use only when there's a strong user need.
Accessibility problems: PDFs are not accessible to some users on mobile devices.
What you should do: if you make a PDF, try to also publish in HTML. If publishing an HTML version isn't possible, publish an HTML summary on the page where the PDF is introduced and provide contact details for users who can't access the PDF.
Making PDFs more accessible:
1. create a logical structure with chapters, headings, paragraphs and sections
2. use bookmarks to help users locate content in long documents
3. use PDF authoring tools that offer accessibility support, like Microsoft Office 10 or Adobe Acrobat’s PDFMaker -- see Create and verify PDF accessibility guide from Adobe

When to use: only when there's a strong user need.
Accessibility problems: Microsoft Word formats (.doc and .docx) don’t conform to WCAG 2.0.
What you should do: if you publish a Word document on the web, provide the information on an HTML page as well. If this isn’t possible, create an accessible PDF and publish both non-HTML formats from a landing page that summarises the document.
How-to guide: Microsoft's guide on making Word documents more accessible

When to use: only publish an Excel document if there's a strong user need. Make sure to provide contact details for users who can't download the document.
Accessibility problems: Excel spreadsheets can be extremely difficult to view on mobile devices.
How-to guide: Microsoft's guide on making Excel documents more accessible

When to use: only publish a PowerPoint document if there is a strong user need.
Accessibility issues: screen reading programs often interpret items on a PowerPoint slide in reverse order, which can confuse users with a vision or reading disability.
How-to guide: Microsoft's guide on making PowerPoint presentations more accessible