A user has an HDD that is almost at full capacity.

To free up space on an HDD without deleting files, using the operating system's compression feature is the best approach. Compression reduces the size of files on the disk by using algorithms to minimize the space they take up, thus freeing up storage capacity while keeping the files accessible.References: File system management, including the use of compression to optimize storage, is a topic discussed in the CompTIA IT Fundamentals, which also covers system utilities and management practices.