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:
create a logical structure with chapters, headings, paragraphs and sections
use bookmarks to help users locate content in long documents
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.