The file was generated using this command:
plink --file pruned_filter_file --make-rel square --out result_file_PCA
the weirdest thing is that the first time i used R the result was great, a PCA plot just like it was intended. Then, in the following times the result was everytime an empty Rplots.pdf. I checked the result_file_PCA.rel and it's a perfectly normal distance matrix, the file is ok. It's weird.