Zotero provides cloud-based storage, see table below. "Zotero Storage synchronizes your Zotero library which includes citations, PDFs, images, etc. automatically among all your computers. This default setting will take most of your storage space* (especially PDFs). If you only want to sync the citations (just plain text) to save space, follow the directions below.
* You are warned when you are approach you free space limit
|Storage Limit||Cost (USD)|
|2 GB||$20/year (equal to $1.67 per month)|
|6 GB||$60/year (equal to $5 per month)|
|Unlimited||$120/year (equal to $10 per month)|