Some PDF files are locked so that printing or editing is disallowed: see list of pages with locked files. But if the file is in the public domain, you can remove that restraint. This page is dedicated to people sharing different ways of unlocking locked PDF files.
You can use this program to split and merge PDF files, but you can also use it to unlock locked PDF files. This method is lossless (i.e. no quality lost). In addition, lossless recompression is done in some cases, with the resulting PDF file being smaller than the source PDF file. (Note: Versions 1.0 and newer of PDFSAM actually obey PDF restrictions, so you will need an older version than that.)
You can simply use the merge tool to merge one single PDF file, and the output file should be unlocked.
Open the file with Gsview. Convert, Device=pdfwrite, Resolution=that of the locked file. A higher resolution will provide a bigger file.
Please upload the file back to IMSLP if possible.
If the unlocking process you went through is lossless and without a large increase in file size, you can upload directly to the same file name, and replace it. Otherwise, please upload it as a separate file (maybe adding "(unlocked)" or "(fixed)" to the file name), and submit it to the same work page as the original file, with a note in the Misc. Notes section of the entry.
Also, if you uploaded the file to the same file name as the original, there may be up to 24 hours delay before the new version of the file will appear on the IMSLP image server, so do not panic if you find the old version of the file still being offered for download right after you uploaded it; wait at least 24 hours before you test it.