This is the tricky part. Adobe does not host version 5.1 on its official website anymore. The official Adobe FTP servers purged these files around 2012. Consequently, you are left with third-party "abandonware" archives.
Many sites claiming to offer "Adobe Acrobat Reader 5.1 free download" are filled with malware, fake "download buttons," and adware installers. Never download .exe files from unknown pop-up sites. adobe acrobat reader 5.1 free download
For older Mac systems (Mac OS 9 or early OS X), archives like Macintosh Repository offer the full installer. OldVersion 2. System Compatibility This is the tricky part
Adobe Acrobat Reader 5.1 is a legacy software version originally released on October 21, 2002 fake "download buttons