Acrobat Reader

Acrobat Reader can be one of the essential tools to have when using Scribus. Although mostly a viewing application, it has some advanced features that no other PDF viewer has, such as full support for JavaScript with a PDF and detailed information about what is embedded in the PDF. Some documents can only reliably be viewed in Acrobat Reader, especially those with embedded 3D objects.

However, Adobe made a lot of changes to the software, some of which may not be to everyone's liking:

• AR is only available for Windows and macOS. The Linux version has been discontinued years ago.
• The installers are huge, because you basically download Adobe Acrobat with many features disabled unless you sign up for an Acrobat subscription. However, Adobe also added many useful features over the years that were previously only available in Acrobat.
• The user interface has changed significantly and faced a lot of critcism.
• Acrobat Reader is "phoning home" a lot, and it's impossible to block automatic and often huge updates.

Depending on your requirements, you may still want to use AR as your preferred PDF viewer.

The alternative: Okular

Fortunately, the Open Source community has developed a more than convincing alternative to AR, namely Okular. Okular isn't a crippled version of a major product focussed only on one or two file formats, but a real document viewer. It can open and display a wide range of file types, including images, eBooks, comic books, and also other DTP export formats like PostScript or XPS. Okular's UI is refreshingly elegant in its simplicity; advanced features are easily accessible via the menu bar.

If you're working on Linux, Okular should be the PDF viewer of choice and is likely to be installed by default – at least if your DE is KDE. Unfortunately, the situation is different on Windows and macOS. As of this writing, the Okular team uses the Microsoft Store as the preferred way of installation on Windows. As an alternative it recommends Chocolatey. There is, however, a standalone installer available, but these installers are always untested "nightly builds". Moreover, it doesn't even provide a portable app.

On macOS the situation is even worse, because you also need to use a "nightly build" – without any alternative.

Haiku

Adobe has never released a version of Acrobat/Adobe Reader for BeOS or Haiku. As an alternative you can use BePDF.

OS/2 and Derivatives

While there are ancient (and today largely useless) versions of Acrobat Reader for OS/2 available for download somewhere, the best choice seems to be Lucide, which is already being shipped with ArcaOS.