Zotero Data Model
Zotero can store any type of file: spreadsheet, presentation, image, audiovisual, dataset, etc. For certain types of files---PDF and other textual documents primarily---Zotero is able to extract or look up bibliographic metadata; for other file types, it simply stores the file with no associated metadata. In the latter case, metadata can be entered manually by creating a parent item for the file.
The parent item that serves as a wrapper around a file is where Zotero stores all bibliographic metadata. A parent item has an item type: journal article, manuscript, book, blog post, etc. The only effect of the item type is to determine which metadata fields Zotero displays; otherwise, any item type can be paired with any type of file. If a different item type would better suit your purpose, feel free to change it.
Metadata Capture Limitations
Zotero's ability to capture bibliographic metadata from PDF files has greatly improved from earlier versions, but it can still get things wrong, and it occasionally produces completely incorrect metadata. Best practice is to manually confirm that metadata is accurate, and if not, to manually correct it.
Snapshots
Zotero's snapshot of a webpage is neither a static image nor a fully functioning webpage, but something in between. It is common to encounter page elements in a snapshot that do not function correctly, or at all. Be particularly cautious of page content that is not rendered automatically when the page first loads, but only in response to clicking a "more" button or link; often clicking the "more" button or link will not work in a snapshot. Best practice is to, after saving a snapshot, verify that the essential content of the page is present. If not, go back to the source website and look for a "print" version of the same page, or use your browser's print function (though that generally suffers the same limitations as Zotero snapshots).