HP Printer Assistant always installs with the printer drivers. So if the HP printer assistant is not installed into your computer, you can download it by yourself by using some steps. Or if you are not able to download or install the HP Printer assistant you can take the help of the technicians. 

For steps you can check out the following link:

https://www.livetechys.com/hp-printer-support-assistant/

For Help in HP printer assistant issues:

Contact: +1-800-214-4506 (toll-free)

For Mac HP Printer assistant is not available. You have to install a new software named HP Printer utility. who have the same feature as the HP printer assistant has. So for Mac how to download HP Printer Utility you can check the same link given.

HP printer assistant is not available for those printers, which are manufactured before 2010. For those printers, you can download the HP Solution Center.