PDF can be useful for a user who is printing the content.
PDF can be password protected within the file.
Reasons to avoid using a PDF
The user is removed from the context and navigation of the website.
Text may be written with a different tone and style and may not be optimized for web.
PDF may not be designed to be read on a screen. Layout and content is static.
Accessibility concerns around navigating the information and usability of PDF with screen readersand other devices
Requires download, demands time and data to acquire information
More time-consuming to update
Mobile and touchscreen experience is not ideal
Suggestions
Put the contents of the PDF directly on a webpage also. Take content out of PDF, whenever possible. It is a much more pleasant experience for website visitors to read the content on a webpage (assuming it has been appropriately styled for web) than to read a PDF. If you must include a PDF, consider providing the same information—perhaps in a more concise manner—on the webpage.
Show the file size and file type when linking to PDF. Provides the user with useful context.
Keep PDF files as small as possible. Browsers often need to download the full file before rendering it. Long download times can mean visitors get impatient and leave the page or site before the PDF appears.
Include navigation in PDF. Include a table of contents, and make sure the page numbers are clickable so users can easily navigate the document.