Sumnotes is described as 'extracts information from PDF files to let you focus on what matters. Features Extract highlights and images' and is an website in the Office & Productivity category. There are four alternatives to Sumnotes, not only websites but also apps for Windows, Zotero and Mac. The best alternative is PDF to X. It's not free, so if you're looking for a free alternative, you could try PdfHighlights or Highlights.
PDF to X is used to convert PDF documents to Word (.doc)/WordPad (.rtf) documents, HTML web pages, JPG/PNG/BMP/TIF/GIF/PCX/TGA images and plain text files in batch mode. It works without Adobe Acrobat or Adobe Reader.
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.
Highlights is a 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.
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.