Translation:It is open until 8 pm.
If you go to https://jisho.org/search/%E9%96%8B%E3%81%8F and click on "show inflections" it shows the ways that the pronunciation of 開くchanges under various circumstances.
This comment also sums up the difference between transitive & intransitive: https://forum.duolingo.com/comment/23369832#25611999
EDIT: May've misread the question. If this is about -te form, @sasuke1 explained this in an above comment.
You should be looking for the verb 開く. Remember the English verb 'open' is ergative whereas the Japanese verb '開ける' is not. So in order to express the intransitive sense of the verb 'open' (or in this particular translation, the adjective form of it), Japanese has a different verb, which in this case is 開く and 開いて is its conjunctive form.
You might want to provide a bit more context in your questions; it wasn't apparent to me whether you're asking about verb differences or -te form, etc. Also, did you not check either of the links provided? The second one should've directly answered what you just wrote, with an explanation and examples of 開くvs 開ける. And the first linked to a dictionary entry that provided inflections for the correct verb for this sentence. I edited my original comment though; I think it sounded a bit short-tempered.
I wouldn't say use one or the other. The time of day is required based on the Japanese text that's there, but the o'clock is not. It shouldn't be counted wrong in my opinion, but I do agree that saying "8 o'clock pm" is a more pedantic style than most people would use. Most would say "8 pm" or if they do use o'clock they'd say something like "8 o'clock in the evening".