Why it's a strong pick

Hypothesis marks up the web where you read it. It works on web pages and on PDFs. It is open source under the BSD-2-Clause license, and its code is public on GitHub. It uses Manifest V3, Chrome's current extension format. The repository's latest release came on 1 July 2026.

What it does

Hypothesis lets you annotate web documents with your Hypothesis account. The manifest describes it as a way to "collaboratively annotate, highlight, and tag web pages and PDF documents." The README says the extension is mostly a wrapper around the Hypothesis client, which appears in a sidebar on the page.

Key features

Highlights and notes. Mark text on a page and add your own comments.
Tags. Label annotations so you can find them later.
Web pages and PDFs. It handles both. A copy of PDF.js is bundled for PDFs.
Sidebar. Your annotations sit beside the page you are reading.
Options page. Change settings in the extension.
Open source. The code is public under BSD-2-Clause.

Who it's for

Hypothesis is for students, researchers and teachers who want to mark up and discuss web pages and PDFs. You need a Hypothesis account to use it. If you want notes that live only on your own computer, this is not the right fit.

What permissions does Hypothesis ask for?

Its manifest asks for scripting, storage and tabs. It lists webNavigation as optional. It also asks for access to all sites. Broad site access is how it can show annotations on any page you open. Read the install prompt before you add it.

Where are my annotations stored?

In your Hypothesis account, not on your computer. The README says the extension works with your account, so your annotations are stored with it. The manifest sets offline_enabled to false, so you need a connection to the service. Plan to be online when you annotate.

Does it work on PDFs?

Yes. The manifest description names PDF documents, and the extension bundles PDF.js to display them. The README does not say how well it handles scanned PDFs, so test one of your own files first. Open it in Chrome and try a highlight before you rely on it.

What does Hypothesis do with my data?

We could not read the developer's privacy policy when we checked; read it before installing. The README says the extension works with your Hypothesis account, so expect your annotations to be stored with that account. We did not find any other data claim in the files we read.

FAQ

Do I need an account? Yes. The README says annotations use your Hypothesis account.

Is it open source? Yes, under the BSD-2-Clause license.

Does it work in incognito? The manifest sets Chrome incognito to split mode. Check how that behaves in your own browser.

Good to know

Your annotations depend on the Hypothesis service. The manifest sets offline_enabled to false, so plan to be online when you annotate. Start with one article or PDF to see how the sidebar works before you use it for a whole project.

Other note-taking tools to compare

Hypothesis is one of 8 picks in our note-taking guide:

Notion Web Clipper: Made by Notion. Save a web page into a Notion page or database in a few clicks. Notion account needed.
Google Keep: Google's own extension for saving page info, selections and images into Keep notes. Free, Google account needed.
Evernote Web Clipper: Clip articles, full pages and screenshots into your Evernote account, then mark them up. Free extension, Evernote account needed.
OneNote Web Clipper: Microsoft's extension for clipping web pages into OneNote. Free to use, Microsoft account needed.
Obsidian Web Clipper: Obsidian's own free extension. It highlights pages and saves them into your local vault as Markdown. Needs the Obsidian app.
Joplin Web Clipper: Clip pages, selections and screenshots into your Joplin notebooks. Needs the Joplin desktop app. Open source.
SiYuan Clipper: Clips a selection or a whole page into your SiYuan notes. Needs the SiYuan app and its API token to work. Source on GitHub (Mulan PSL v2).