Highlights is described as 'PDF reader for note takers. Highlights lets you highlight text and select images in the PDF, but take notes in a regular text-editor. No more typing notes and comments into tiny text boxes' and is an app in the Office & Productivity category. There are five alternatives to Highlights for a variety of platforms, including Online / Web-based, Mac, Windows, iPhone and iPad. The best alternative is ReadCube Papers. It's not free, so if you're looking for a free alternative, you could try ZotFile or PdfHighlights. Other great apps like Highlights are MarginNote (Paid) and Sumnotes (Free).
Integrating powerful tools for book annoation, mindmapping, flashcards and more, to build up your reading notes as never before. Start Your Reading - Document vs Study Document for book management and light read-annotation; Study notebooks with a full set of toolkit.
Zotfile is a Zotero plugin to manage your attachments: automatically rename, move, and attach PDFs (or other files) to Zotero items, sync PDFs from your Zotero library to your (mobile) PDF reader (e.g. an iPad, Android tablet, etc.) and extract annotations from PDF files.
PdfHighlights easily extracts and exports all your highlights and annotations from all your PDF files into a single, clean report. PdfHighlights is available both as a cross-platform HTML5 web app and as a Windows application.