Earlier this year, the Nielsen Norman Group repeated a study they first did in 1996 on the usability of PDF documents. As they've now found three times, making PDFs instead of actual web pages yields a horrible experience for users:
Jakob Nielsen first wrote about how PDF files should never be read online in 1996 — only three years after PDFs were invented. Over 20 years later, our research continues to prove that PDFs are just as problematic for users. Despite the evidence, they’re still used far too often to present content online.
PDFs are typically large masses of text and images. The format is intended and optimized for print. It’s inherently inaccessible, unpleasant to read, and cumbersome to navigate online. Neither time nor changes in user behavior have softened our evidence-based stance on this subject. Even 20 years later, PDFs are still unfit for human consumption in the digital space. Do not use PDFs to present digital content that could and should otherwise be a web page.
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. If they knew these, they’d probably just create a web page in the first-place, not a PDF. As a result, users get stuck with a long, noninclusive mass of text and images that takes up many screens, is unusable for finding a quick answer, and boring to read. There’s more work involved in creating a well-written, accessible PDF than simply exporting it straight from a word processing or presentation platform. Factors such as the use of color, contrast, document structure, tags, and much more must be intentionally addressed.
Yah, so, don't use them.