is it safe to download google chrome onto my imac?

Yes, it's definitely safe to download Google Chrome onto your iMac. I have it on my MacBook and it's a very good browser. Also about Adobe flash, there is already a version of Adobe flash that comes preinstalled on all Macs.

- Daniel 

You may not need to download Adobe Reader as Apple's pre-installed app Preview copes with a variety of file formats, PDF included.

If you do download Flash it is best to only ever get it direct from the Adobe site: http://get.adobe.com/flashplayer/

As DanielPro has mentioned, if Chrome was to become your default browser you would not need the separate install of Flash because it is integrated into Chrome.

You can install as many web browsers as you like, but you probably don't want them all running at the same time so they don't hog your RAM.

The link I gave you is to the Adobe site, the only safe place to download Flash.

To summarise - if you only used Chrome you wouldn't need a separate installation of Flash and could uninstall it using Adobe Flash Player Install Manager which is located in your Utilities folder.

Of course, if you do uninstall it you wouldn't be able to view Flash content if you decided to use Safari or any other browser that doesn't have Flash built-in.

To stop Flash downloading automatically go to System Preferences>Flash Player and reeview your settings under the Advanced tab. I have mine set to 'Notify me to install updates' - this ensures that you keep up-to-date with Flash, but you should always be wary of what you're installing - don't just update without verifying what you're about to do.