How much do you want to spend on a bike?
A Pacific bike can usually be found at Walmart or department stores. It is not a bike designed to be used for actual mountain biking, but rather for riding around the neighborhood and *maybe* unsealed fireroads. Assembly will be the biggest concern, as they'll usually be put together by untrained low-paid assemblers, not skilled bicycle mechanics. You'll get no service backup from the retailer. It's a bike that might be good to get you interested in cycling, but if it is assembled as poorly as some that I have seen it could just as easily turn you off cycling for good!
These bikes are usually offered in one size only, and when it comes to bikes one size does not fit all! If it does happen to fit you and you do decide to buy one, at the very least take it in to a dedicated bike shop and have them give it a good look over from an assembly and safety standpoint.
My advice? Save up about $300 and go to a dedicated local bike shop (LBS). There you'll get a bike with a strong, quality frame in the size that fits you. It will be assembled by a trained bike mechanic and come with service backup (often free lifetime tuneups). Good options here are Giant, Trek and Specialized.
Good luck!
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