If it's a Kindle Fire, it's running Android. Sideload a different e-book reader app. I suggest using Moon+ Reader. That's what I use for epubs. There's the link, if you want to get the direct APK.
Looking up the pro version it doesn't sync...
"Annotations, highlights & bookmarks share support
Open/Backup/Sync/Download/Uploald book files via Dropbox/GDrive"
Are the only major differences.
Edit: FAQ has a back up syncing from Dropbox. Not sure if it's pro only. Looking it up it looks like a pro feature.
Most of my ebooks whether paid or free come from Amazon. I maintain my collection in Calibre and recommend Moon+ Reader for actually reading from, especially if you want to use TTS functions.
You can play with the free version and purchase pro if it's the app for you. the official site offers a lot of information on the app and the differences between the two
tldr; ipads much better tablets, pixelbook much better laptop. Also ipads win for offline use, much better at syncing stuff/media, and there are a bunch of productivity apps which work offline well.
Pixelbook works great as a laptop and limited tablet use for web browsing.I really like how light and fast it is for web based stuff. I also use it for linux apps, works decently.
but, it really doesn't work well for android apps as it could/should. For example I use Moon Reader to read and highlight pdfs on the pixelbook but the experience is not good and its never fluid, and here I'm comparing it to using a original ipad air from work.
So, I'm considering getting my own ipad at some point for pdfs and so on.
Originally when I got the pixelbook, the dream was that it could be a 2-in-1 - which it is, but in practice the weight and not so nice android and missing and half implemented touch gestures and so on make it a tablet fail.
when you open moon reader, it freezes
I couldn't use it for a couple of months until I discovered this note on the official faq:
http://www.moondownload.com/faq.html
that's why I had to use other readers without infinite scroll
I use this android app to sync a few dropbox folders offline.
At the moment I just save them as subfolders in the Downloads folder, then use MoonReader+ to view pdfs etc.
Though I'm not fully sold on MoonReader yet, but its good enough.