You have an unlimited amount of metadata storage. You can store many thousands of Zotero records.
The 300 MB relates to the size of the files that you wish to store. For example, if your PDF files are each 2 MB you can store about 150 of them. If your PDF files are 0.5 MB each you can store about 600 of them.
Consider that Zotero file storage is an important source of funding for the project. I pay for unlimited storage but use only a small amount of storage because that is a way for me to financially support this project.
Upgrade Storage
The 300 MB relates to the size of the files that you wish to store. For example, if your PDF files are each 2 MB you can store about 150 of them. If your PDF files are 0.5 MB each you can store about 600 of them.
Consider that Zotero file storage is an important source of funding for the project. I pay for unlimited storage but use only a small amount of storage because that is a way for me to financially support this project.