There has never been a free version of MATLAB; use of the software requires a purchased license. If you own a legitimate license for R2007b, you can retrieve your activation details through official channels:
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MATLAB 2007b, also known as MATLAB 7.4, is a significant release of the MATLAB software. It was introduced in 2007 and brought several enhancements and new features, including improved graphics, new data analysis tools, and enhanced support for parallel computing. MATLAB 2007b is widely used for tasks such as data analysis, algorithm development, and modeling and simulation.
The "PLPTXT" file might be related to a license file or a patch used for licensing purposes.