Am working on a very large dataset consisting of different files, each file have different length. have already select the useful variable for the analysis but to merge all the files together is given me problem.

all the files are meant for the same purpose but due to entering error, they are of different length, for instance, A file have up to 11900 observation while B file have just 3456 observation.

I need to merge all the files together in order to have a subset of the whole original data set.