Are you referring to a file that you've saved to your Acrobat.com online account? If so, you'll need to download that file to your computer to edit it. Please follow these steps:
Select the file that you want to edit, and then click Download. The file will be downloaded to the Downloads folder on your computer. From there, you can open it in an application that supports that file format and edit it.
Please note, that if you're working with a PDF file, you'll need to use Acrobat to edit the content. If you don't have Acrobat, please feel free to download a 30-day trial from http://www.adobe.com/products/acrobat.html.
I use Adobe Reader application on my iPhone. I can edit files with highlighting, writing comments...etc and save that immediately. Is there anyway I can do the same thing when I upload the files on cloud.acrobat.com without having to download them to my computer?
When you have files saved to your Acrobat.com online account, you can use the File > Open from Acrobat.com option in Reader to open those files, and then comment on them on your desktop. Then, you can choose File > Save to Acrobat.com to save them back to your account.