Some time ago, Facebook started building their own web browser into many versions of the Facebook App. The idea was to keep people inside the Facebook system for longer, but the In-App version of the browser has a number of restrictions and does not fully support all the features available in modern stand-alone web browsers. This can affect how many websites work, including this one.
The In-App browser is designed to discourage you from leaving so that Facebook can track what you do on any website link that you open when using the App. This gives them more data about you and your likes and dislikes and a greater chance to feed you adverts from 'relevant partners'; that is, advertisers that pay Facebook for the privilege.
However, the restrictions this imposes mean that, amongst other things, you cannot open links in a new tab or window and you cannot view or open documents such as .pdf, .doc, etc. using the In-App Browser.
The answer is to set the Facebook App to open links in a standard browser. To do this on Android mobile apps, tap the Facebook menu icon in the top right, scroll down to Help & Settings, tap App Settings and enable the Always open links with external browser option. That’s it.
Next time you try to open a link, Facebook will ask you which app to use and you can pick your preferred browser. Select Always if you don’t want to choose every time.
For iOS, there is currently no permanent option, but you can open individual links from the In-App browser in Safari by tapping the 3 dots (...) or Share option and choosing Open in Safari from the menu.
On the Windows 10 Desktop App you don't have a permanent option either, but when the In-App browser opens, you can select the 'gear' Settings symbol at the top right and then click Open in Browser.