Website visitors are busy. They have questions and are looking for answers. Some visitors are prone to use navigation and links to find the answers, others are more likely to search.
They all tend to scan web pages, not read them, at least until they have found the content they’ve been looking for.
PDFs often do not have links to the primary website for related content and, in fact, often do not contain links within the text at all. And if the PDF links out to a website or another document, the visitor is asked if they want to execute the link and whether this is their preference for all links. Answering this question incorrectly can further isolate the content and negatively impact the user experience.
Some PDFs are large enough to warrant their own Table of Contents which, in effect, results a mini-website. Only this type of "navigation" is often not clickable and this version needs structure applied, remediation and potentially translation.
Due to the inherent remediation effort, updating PDF content to keep it current and relavent is often avoided, impacting the quality of the content which negatively impacts the user experience further.