Viewing PDF comments
Well it seems Adobe has turned its back on the Linux community so I have to update this answer. I suppose you have moved to the latest Ubuntu or at least one that can use the more capable open source applications below.
Viewers With Annotation Viewing Capabilities
Open source viewers
Okular
A KDE based application, runs ok and looks ok in Gnome, one of the best open source applications that can both view and create annotations, note that this application can do standard and non standard annotations. You can create annotations that can be viewed by all viewers that support the pdf standard for annotations.
you can create annotations that does not interfere with the document, you will need to use Okular to view these, these can then be converted to standard annotations if you need to.
Evince
The default viewer on Ubuntu and most Gnome based desktops, the latest now have a somewhat limited support for standard pdf annotation viewing.
Proprietary Solutions
PDF Studio Pro
A very good fully capable PDF viewer and editor, from Qoppa Software, there are trial versions available for standard and pro, the pro verson has most of the features of Expensive tools from Adobe.
Some notable features of the pro version- Redaction, block out parts of documents you do not want people to see.
- OCR, Make scanned pdf searchable
- Digital Signature, sign documents adding image of signature.
- Create and fill forms
- Measure Tool, measure using scale factors, CAD type dimensioning.
Kami (Formerly Notable PDF)
Kami is a browser based solution, can view standard annotations, can create non standard annotations, this works with Google Chrome browser, you can save annotated PDFs on Google Drive read PDF documents from web pages,and download to desktop or save to Google Drive this can replace Chrome's default PDF viewer, there are free, premium and Work versions available the premium version is $15.00 a year and the work version is $50.00 a year.
Some work features- Ocr, create searchable pdf.
- Unlimited e-Signature
- Unlimited Files to Split
- Unlimited Files to Merge
To view comments in PDF files under linux you may consider using current version of Firefox. Just drag PDF file into browser window or put absolute path to given file in the location bar (ie. /home/username/Documents/file.pdf) and hit Enter.
If you are using older version of Firefox it's possible that this feature is disabled by default (since it sometimes caused problems; howtogeek.com has an article that describes how to enable it). You may also install it as addon from:
https://addons.mozilla.org/en-US/firefox/addon/pdfjs/