You do not need to create an account to install or use Zotero. However, creating an account allows you to view your "library" online when you are away from your main computer and sync your "library" between two or more computers. To register for an account, visit https://www.zotero.org/user/register/
Note: Creating an account will not automatically store your PDF attachments online or sync PDF attachments between computers. If you are syncing between multiple computers, it is a good idea to run the sync before you exit Zotero on each computer and again first thing when you open Zotero on each computer. Otherwise, you may not have matching libraries.
With the release of Zotero version 4.0, it is now possible to choose which directory Zotero uses when you attach links to files. If you use Dropbox (or another similar file-syncing service), you can now save your citations in Zotero, attach links to files in a Dropbox folder, and have both synchronize with every computer you use. By doing this, all of your citations and saved electronic documents will be easily available where you are working, as well as being backed up in the cloud and your hard drives. Here are instructions to set up the sync and link files:
1) Sign up for a Dropbox account. 2GB of storage is free. 100GB is available for $99/year. [For cheaper options providing large amounts of storage, Google Drive offers 5GB free and 25GB for $30/year and SugarSync offers 5GB free and 60GB for $75/year. If you go with Drive or Sugarsync, just substitute them for Dropbox in the file examples below.]
2) Download or upgrade to Zotero 4.0. I recommend downloading the Standalone version and then installing the appropriate browser extension for Chrome, Safari, or Firefox.
3) Create a folder within Dropbox to store all of your saved digital documents such as PDFs of articles. (In my example, that folder is labelled "Zotero.")
4) In Zotero, change the setting for the Base Directory to the Dropbox/Zotero folder. The path is Preferences/ Advanced/ Files and Folders. Here's what it should look like:
5) Move all your saved files and folders with digital articles and ebooks to the Dropbox/Zotero folder. You can keep whatever folder organizational structure you have already and just move everything into the Zotero folder.
6) In Zotero, right-click on a citation for which you have a saved digital file, then choose Add Attachment, then click Attach Link to File. Find the appropriate file in the Dropbox folder and attach it.
7) Once you've attached the link to the file, as long as the settings are correct and the same on each computer, Zotero and Dropbox should synchronize with whatever computers you use, and the file should be retrievable through Zotero.
Zotero offers online storage, allowing you to sync your library so that it is available on any computer if you log in to your Zotero account at zotero.com. You'll automatically receive 300 MB of storage by signing up for a Zotero account. The first 300 MB of storage is free. After that, pricing for storage is:
To upgrade your storage, click on the blue "Upgrade Storage" button at the top right of any zotero.com page. For more information, see https://www.zotero.org/support/storage