There is a newer resource on Understanding Accessible PDFs that is the current official summary of our knowledge and services around PDF accessibility.
It is usually overall more accessible to make documents into accessible html documents, not have them be external documents, but instead integrate them into a site or application. However, if that is not possible, it is important to consider the accessibility of all external documents with links from your site or application.
Microsoft Word Documents
Here are some core principles on creating an accessible Word document by WebAIM.
• Use true numbered and bulleted lists.
• Ensure that font size is sufficient, usually around 12 points or more.
• Provide sufficient contrast.
• Don't use color as the only way to convey content.
• Use true columns, not tables or columns created by hand with the Tab key.
• Provide a table of contents, especially for long documents. If you have a good heading structure, creating a table of contents is easy.
• Use simple language.
This Microsoft Office help article on Word covers most of what is necessary to make a Word document accessible to blind and low vision users in more depth, describing how to do it. All of the points they cover in this article are relevant to visual impairment except the last one.
One more point the previous articles do not cover is defining the language. Defining the language is mostly useful if you have more than one language in your document, but it’s also helpful to someone who is accessing content in more than one language on their computer. For example, if the language is not properly defined, a screen reader might try to pronounce English words according to Russian rules for pronunciation. This article from the US Department of Veterans Affairs explains how to set the editing language.
Converting from Word to PDF
Some accessibility features, such as headings, lists, and tables, are maintained when you convert a document from Word to PDF, while others, such as document language are lost. This Office help article explains how to create a properly tagged PDF from a Word document.
PDFs
This WebAIM article on PDF accessibility takes a slightly more introductory approach, although it covers an extensive set of topics.
This article by Adobe on accessible PDF’s provides a thorough explanation on how to create and check for an accessible PDF with Acrobat Pro.
Conclusion
External documents are an integral part of a website or application, so it is important to make them accessible.