Why does pdfreaders.org recommend only Free Software PDF readers?

pdfreaders.org recommends only Free Software PDF readers because those programs respect the four basic freedoms — to use, study, share, and improve the software — which the site argues gives you control over your own computer and helps protect your privacy. The site frames this as a deliberate choice rather than a neutral software directory: it also warns that proprietary alternatives, including Adobe's reader, keep their internal workings as trade secrets and "do not respect your right to control your own privacy and data."

The two arguments behind the recommendation

The site makes two distinct points, and it helps to keep them separate.

1. Software freedom as user control. Every reader on the list is Free Software, which the site defines by the four freedoms: use, study, share, and improve. The claim is that this legal and technical status is what gives you control over your computer and protects your privacy — not any specific feature.

2. Open Standards as a separate concern. The site also discusses PDF versions, noting that some qualify as Open Standards, some are ISO-certified, and some are "encumbered by software patents." It argues you may want to promote the Open Standard versions because Open Standards "guarantee interoperability, competition and choice." This is about the format, not the reader — a Free Software reader and an Open Standard PDF are two different things the site cares about.

What the site actually lists

The recommended readers, with the platforms given on the page:

A few details from the page worth noting:

• Xpdf carries a caveat. The site states that "the XpdfReader builds are non-free, we advise against them" — so the project is listed, but a specific distribution of it is not endorsed.
• sioyek is described as designed for reading research papers and technical books, which makes it the most task-specific entry on the list.
• Okular and Evince are the document viewers of the KDE and GNOME projects respectively and handle formats beyond PDF.
• SumatraPDF is described as powerful, small, portable, and fast to start.

Practical consequences if you follow this advice

The site adds an installation note that matters more than the list itself: most Free Operating Systems provide package management systems, so you generally do not need to download files manually from project pages. Installation instructions live on your distributor's website. The download pages linked from pdfreaders.org provide source code if you want to build the software yourself.

So the practical path is: identify your platform, pick a listed reader, then install it through your distribution's package manager rather than chasing a binary from a project page.

When this recommendation may not fit you

The site's logic is coherent, but it is a values-based filter, not a feature comparison. If your priority is a specific capability — say, advanced annotation, form filling, or a particular accessibility workflow — the list does not rank readers on those grounds, and the page does not claim to. The recommendation answers "which readers respect my freedoms and privacy?" It does not answer "which reader is best for my document workflow?"

The site also invites contribution: it asks readers to help translate the interface or download pages of their favorite PDF reader, which signals that the list is maintained as a advocacy resource rather than a comprehensive catalog.