See screenshot. The first tab takes over a half of the tab bar, and the last two are ellided after just 2 letters. Reproducible: Always
Created attachment 99185 [details] screenshot
Which version are you using?
16.04.1 aka 0.25.0
I can confirm this issue. The following screenshot is from a virtual machine with Fedora 23. There are two okular programs open in this screenshot. The first one in the background is the stock okular, I opened the longname pdf first, then the shortname pdf second to confirm as the bug report states. The okular running in the foreground was launched from within Kdevelop, with a two line code change. Seems to work properly to resolve the issue.
Created attachment 99276 [details] Snapshot of the problem as well as results of code change
*** Bug 381875 has been marked as a duplicate of this bug. ***
Confirmed this is still an issue on Okular from git master. Antonio if you're still interested the code is now at https://invent.kde.org/graphics/okular/-/merge_requests
*** Bug 429844 has been marked as a duplicate of this bug. ***
Making all tabs stretch to fit would effectively fix this. Konsole now takes that approach and it works well.