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My paternal grandfather was raised by others and took their name, but he was not officially adopted. I have no idea of his real name.
Can I still find him/family with DNA test(s) and, if so, which one(s) would be best to do?
There is no one alive that has any information.
Yes, DNA testing could be very helpful, although it won't give you a direct answer.
Have an autosomal DNA test done of the oldest generation still living, your grandfather if he is still alive, otherwise one of his children, otherwise you. Meanwhile, build out a family tree of known relatives. For example, if you get DNA from your father, also build out a family tree of your father's mother's family.
When you get DNA matches, ignore them if they are on the side of your known family. what you should pay attention to are DNA matches of unknown relatives. You should then contact these relatives, share what information you have, and ask if they can help. Even if they don't respond, if you have their name, you can start building their family tree using ancestry.com or my heritage.
DNA tests will also be useful for figuring out your paternal grandfather's ethnic heritage, which will affect the search. For example, if he was of European Jewish descent, your search will be harder because of endogamy, which will incorrectly show more close relatives than possible. In my experience, different sites are popular with different ethnic groups. There seem to be a lot more Jews on 23andme than an Ancestry. (I'm Jewish.)
23andMe recently added a nice family tree tool to help you triangulate different matches, but you can use free tools to do the same thing, although it will take work.
Even though Y-DNA is theoretically most useful for determining paternal lines, not enough people have been tested for it to be likely to be useful, or at least that's my experience.
If you can post more information about your paternal grandfather, such as what is known of is ethnicity and where and when he was born, and about the relative that would be tested, that would be helpful, as would your budget and whether you are interested in doing things by hand or want the simplest possible technical solutions.
What you suggest is eminently doable, so much so that the techniques are well known. In your case, you're working from descendant DNA, which makes for a more complex elimination problem, but you also have foreknowledge of e.g., geographical location, that can help eliminate many matches.
Based on "mystery" matches within my own tree, you will be served enormously well by finding a relative in your grandfather's tree who has already researched his biological family. Such a relative might be identified via matches with some member (or members) of your grandfather's family. The likelihood of finding such a relative, though, is extremely uncertain.
AncestryDNA has by far the largest pool of autosomal DNA samples, but their tools are poor (e.g., no chromosome browser, no triangulation), and many users have no useful trees and/or don't respond to messages (non-response is an issue with all vendor sites). I would still receommend it as the best place to start. MyHeritage also has a large sample pool, a higher percentage of whom are outside the U.S., if that's a factor. Also, their tools are excellent, and samples from other vendors can be uploaded there. 23AndMe has a smaller sample pool, but also has excellent tools, with the notable exception of their support for user trees (most charitably described as arcane).
Eliminate matches from your other ancestral lines, and identify one or more likely ancestral families for your grandfather. Then, identifying a DNA-match relative in such family with a well-documented tree could prove enlightening, as they are more likely to know how the shared DNA matches fit in that tree (even if those others have no trees themselves).
Having identified (or constructed) a candidate tree, you can then use the DNAPainter WATO tool to determine where your grandfather is likely to fit:
https://dnapainter.com/tools/wato
CeCe Moore makes this look easy & quick, but it generally isn't. Be patient, it could take months, or years, especially if there aren't enough closely-matching samples to work with (you may need to wait for them to show up).
