Research spanning 20 years proves PDFs are problematic for online reading. Yet they’re still prevalent and users continue to get lost in them. They’re unpleasant to read and navigate and remain unfit for digital-content display.

Given PDFs poor usability for online reading, user-experience designers should either avoid using PDFs altogether in favor of presenting content on web pages, or, in cases where a printable PDF is needed, use an HTML gateway page. Gateway pages are web pages that summarize the key points and critical information from a PDF, and then they offer users the option to download the full PDF without having to open it in a browser window, where it’s unfit for consumption. The PDF file size should be as small as possible to preserve quality, while allowing it to download quickly.

Some of the usability problems that PDF files cause on websites or intranets listed here are:

1. Linear and limiting:  PDF files are typically converted from documents that were planned for print or created in print-focused software platforms. When creating PDFs in these tools, it’s unlikely that authors will follow proper guidelines for web writing or accessibility
2. Jarring user experience: PDFs look completely different from typical web pages. They take users out of a familiar context and into one that is outdated and clunky. 
3. Slow to load– While the risk of crashing a user’s browser or computer by serving up a PDF is lower than in years past, PDFs can be excruciatingly slow to load both on desktop and mobile. In one recent study, a cafeteria menu housed in a PDF on an intranet took almost 3 minutes to download.
4. Cause disorientation: Because PDFs aren’t web pages, they don’t show a standard navigation like a website would. 
5. Unnavigable content masses.  Most PDF files have no internal navigation. Users often struggle to stay oriented with where they are, and how to get back to where they were before.
6. Sized for paper, not screens: PDF layouts are often optimized for a printed sheet of paper, which never aligns with the size and scale of the user’s browser window. Regardless of whether they’re viewing the PDF on desktop or mobile, users can say “so long” to smooth scrolling and hello to tiny, unreadable fonts.

https://www.nngroup.com/articles/pdf-unfit-for-human-consumption/