I looked at Skim to see if it was a useable and easy PDF reader for my laptop.
The really good thing about it is there are two sidebars - one which shows pages of the file, and another that shows every edit, note, highlight, strikethrough, etc. on the file in page order. I thought that this would be great for reading and annotating long texts, or even short texts with lots of annotations in a small space that may get lost amongst each other.
One downside is that annotations cannot be exported as a .pdf and read in Preview, nor in iBooks. As a PDF bundle it works, but only opens in Skim. The ‘notes as text’ functionality exports: highlighted text, underlined text, boxes, anchored notes, and circled passages in the .pdf text, along with the page number the note was made on. The result is the same whether the exported notes are saved as .rtfd or .txt. There is also a script to export notes from Skim into Markdown, which could then be read alongside a copy of the PDF.
After using it to read three different texts (two long ones, and one article a few pages long), I would say that I would probably use it for big readings, but not for more casual text or articles.