Viewing PDFs natively within applications used to be a major headache for developers. Even though the PDF standard has been open and available for decades, most people still relied upon external reader software and browser plug-ins to open, view, and edit PDF files. Given the complexity of PDF rendering and discrepancies between readers, there was no guarantee that two people viewing the same document would see the same thing.
When Mozilla developed and released PDF.js as an open source library in 2011, developers finally had a way to easily integrate PDF viewing into their web applications to ensure a consistent viewing experience. Even then, however, this JavaScript library didn’t support the entire PDF specification and lacked key features like annotation tools. That may not have presented much of a problem ten years ago, but today’s users no longer regard annotation as a “nice to have” feature. If your web application doesn’t allow them to natively markup or sign documents, they’re liable to turn to more comprehensive solutions pretty quickly.