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.
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.