I have a high PR (5) Travel website.I want to do link building for this site.I want to know that is forum posting on irrelevant High PR forum like Digital Point Forum,going to help.
It will help simply because you get to choose the anchor text, but it obviously won't have anywhere near the same impact you would get from a closely related site.
Cause of Anchor text in the signature, links from authoritative non-relevant forums works well for targeted keyword ranking but they don't carry as much weight as relevant forum. Also, if you post in relevant forums you can improve both rankings as well as usability.
I'd say every link helps towards gaining PR, but only relevent links help to gain traction in the serps.
I suppose that at some point the search engines are going to do some algorithm update and don't take into account any link from any unrelated forum anymore. That would make sense, because having tons of links from unrelated forums doesn't tell a thing about the relevancy or quality of a site.
There must be better things to do with your time. Concentrate on finding travel related forums and post there instead.
I think irrelevant links work if they are from good trusted sites. If you can arrange a link on such a site, get it, no matter its irrelevant to your topic.