Zotero gives each user 300MB of file storage for free. If you need more space, you can:
1. In the OneDrive folder on your computer, create a Zotero attachments folder.
3. Download and install Zotfile. This is designed for the Firefox version of Zotero, but will work with Standalone--just follow the directions on the Zotfile homepage under "Install Zotfile."
4. Open Zotero. Go to Tools > ZotFile Preferences (or gear icon > ZotFile Preferences). On the General Settings tab, under 'Source Folder', choose the default location (C:\Users\[username]\Zotero). Under 'Location of Files', select Custom Location and choose the Zotero attachments folder you created in OneDrive.
4. On the Tablet Settings tab, check "Use Zotfile to send get files from tablet". Then choose your attachments folder in OneDrive as your base folder. Click OK.
5. Close the Zotfile Preferences window and open Preferences from Zotero's Edit menu. Go to the Advanced menu, then select the Files and Folders tab. Make sure that the Base directory field is blank.
6. Finally, in the middle column of your library, right click (or command-click) on one or multiple references with attachments and select Manage Attachments > Send to Tablet. The next time OneDrive syncs, your PDFs will sync as well.
Zotero gives each user 300MB of file storage for free.
Citations take up little space. PDFs, snapshots, and other attachments take up lots of space.
Moving your Zotero library to Dropbox, Google Drive, or other cloud storage will irreparably corrupt your Zotero library. Instead, you can use these services to store attachments using Linked File Attachments.