I've been using a Chromebook as my sole productivity machine for about a month now after introducing Chrome OS to my workflow over a year ago. While I've been surprisingly happy with the experience, it took me ages to find the perfect PDF reader to replace the excellent PDF Expert app on macOS. I ended up testing quite a few tools and created a fine selection of solutions that might help you find your preferred workflow for your PDF needs, too.
I personally need a PDF reader to annotate film and TV scripts mostly, but your workflow might differ from mine. That's why I won't only introduce my personal go-to solution, Kami, but also a few other alternatives that might be better suited for other use-cases.
Before we dive in, we need to establish a vocabulary to make it easier to differentiate between the different tools. Editing a PDF is a process similar to working on a Word document — you can change the text, switch out images, and add new elements to a page, altering it completely in the process. Annotating a PDF can be compared to taking notes on a printout. You can underline things, add shapes, forms, and text, but you can't change the underlying PDF document itself. Finally, signing and filling out a PDF is much like signing and filling out a printed document, though PDF files allow for special forms and boxes that you can manipulate via a viewer.
The native Chrome PDF viewer
If you don't need much more than a basic PDF viewer with rudimentary support for annotations and signatures, you might not have to look further than Chrome's native PDF viewer. It's the default solution for opening PDF files you've downloaded on your Chromebook, and it's more than enough for many circumstances. If you have a stylus, the viewer makes it easy to underline, draw, sign, and write with a selection of tools, but things get a little more complicated once you'd like to draw straight lines, shapes, or just add text — the Chrome PDF viewer currently doesn't support that. You also have to be careful about saving changes made to the documents you annotated or signed by hitting the download button in the top right corner, as there's no auto-save option. At least the PDF reader is able to fill out forms in PDFs.
Despite these shortcomings, the built-in PDF reader is great if you don't want to deal with downloading an app or signing up for yet another online service, and it doesn't cost you a dime.
Online tool: Kami
Over the last few months, Kami managed to become my go-to solution for PDF annotations. The web app connects to Google Drive, Microsoft OneDrive, Dropbox, and Box and pulls your documents straight out of them, but you can also upload PDFs from your local storage. You can even use the app offline if you install its companion Chrome extension, though you won't be able to download the documents you're working on while offline. The annotation tools are as intuitive as they can get, and you can even assign custom keyboard shortcuts, making it easy to go through a text without Kami getting in the way. Like any good PDF reader, Kami supports miniature page previews in the sidebar and an annotation overview.
Since Kami is mostly used in educational contexts, the tool is extensively scrutinized and tested for privacy. It looks like it excels in that regard, according to the Common Sense Privacy Program, so even if it's an online app, your potentially sensitive documents should be safe.
While the free version of Kami is great for annotations, you'll have to pay for the $5/month (or $50/year) Pro plan if you need more advanced tools. It's required if you want to add signatures or images, but the plan also gives you access to a built-in dictionary, a text-to-speech engine, equations, options to add more pages or to merge and split existing files, advanced export options, and more.
Keyboard and trackpad warrior: Xodo
Many Android PDF readers that do the job just fine on Chromebooks, but I personally found Xodo to be the most robust for Chrome OS. It has a decent interface that works well enough with a trackpad and keyboard, and there are lots of annotation options and a free signature tool. It's also one of only a few PDF readers on Chrome OS and Android that allows you to merge, separate, and reorder sites in a PDF file for free, though reordering is tough to do without a touchscreen.
My biggest gripe with Xodo is that it doesn't integrate well with Google Drive and OneDrive, though that isn't necessarily the app's fault — both Google and Microsoft no longer maintain the necessary APIs, so Xodo chose to remove the option to connect with Drive and OneDrive instead of relying on a system potentially prone to breakage. You can still open files from OneDrive and Drive via the system file picker.
I also looked into PDFelement (it hides more features behind a paywall than Xodo, doesn't support text annotations, and doesn't work very well with a mouse/trackpad) and Foxit (it crashed on me in the middle of a script reading session and didn't save any changes).
Pen and paper: Squid
If you're someone who has a Chromebook with an active stylus, Squid might be an even better option for you. It's an Android app focused on making note-taking simple by emulating a pen-and-paper experience, and it's one of the most polished productivity apps in the Play Store. The only caveat — you need the Squid Premium subscription or a one-time in-app purchase to unlock PDF import functionality. Cloud storage is similarly locked behind the subscription (or another in-app purchase). However, most newer Chromebooks come with a free 6-month trial of Squid Premium, so you can extensively test if the app works for you without paying anything upfront.
Great for signatures: Adobe Acrobat Reader
The Adobe Acrobat Reader is another capable Android app for annotations and likely your best bet if you just need a tool for signatures and filling out forms. To get started, you first need to sign in with an Adobe ID, which you can create right from the app if you don't have one already. You can then annotate and fill any PDF file you have on your Chromebook for free. Signatures are really intuitive here, too. You can save your signature to the Adobe Cloud and reuse it whenever you log in to your account on a new device, and the app is also compatible with Google Drive, OneDrive, and Dropbox.
Advanced features like editing, rearranging pages, password protection, and more are only available as part of Adobe's Premium tools for mobile subscription. On Android, I'd also recommend the Adobe Fill & Sign app that's just perfect for quickly filling out forms, but I found it to be pretty clunky to use with trackpad and keyboard. You could give it a try if your Chromebook has a touchscreen, though.
These are just a few options that should cover most of your needs when it comes to PDF files. Each has its strengths and weaknesses, and you might find yourself liking one tool better than another depending on what you need to do with PDF files. Many people should be fine with the built-in Chrome PDF viewer, but if you do need more advanced features, the Acrobat Reader should cover most of your needs without making things too complicated. And when you need to go all in, Xodo and Kami are the tools to turn to.