dtSearch. Although it costs something (don't know what, I bought it several years ago), the upgrades are free and the program is easy to use with very good searching capabilities.
It does merge with Adobe Acrobat and will highlight search findings in the pdf itself, but somehow I screwed that up and now it doesn't work that well with Adobe. Doesn't matter because you can get the results you want, with a lot of context around each finding, so that you get what you really want anyway.
I have used Archivarius, which is free, but it doesn't have the power of dtSearch.
Don
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