Download & Install CS5.1

That version is not compatible with a modern Mac

1. you can install the latest and 1 version prior versions using the cc desktop app. you must have a subscription because previous versions are not available with trials. other versions are revealed after clicking file preferences>apps>tick show older apps (at panel bottom).

2. for versions cc 2019 and older, use prodesigntools.com*.

3. for versions newer than cc 2019 but older than one previous version, contact adobe support for links**.

---------------------------------------------------------------
* most (but not all) adobe download links are available from prodesigntools.com. https://prodesigntools.com/adobe-direct-download-links.html

follow the directions (esp, step 1)

save the install file. it may not remain online.

thank jim @ prodesigntools.com
-------------------------------------------------------------

** this support is currently not available. ie, there's a significant gap (between 2019 and last year) in the available previous versions.

opposition to this decision (to limit available previous versions) has been conveyed to adobe officials who are therefore aware of the problems caused by this policy. however, unless/until changes are made, nothing can be done to legally access these files.