Zotero (/zoʊˈtɛroʊ/) is a free and open-source reference management software to manage bibliographic data and related research materials, such as PDF files. Features include web browser integration, online syncing, generation of in-text citations, footnotes, and bibliographies, an integrated PDF reader and note editor, as well as integration with the word processors Microsoft Word, LibreOffice Writer, and Google Docs. It was originally created at the Center for History and New Media at George Mason University and, as of 2021, is developed by the non-profit Corporation for Digital Scholarship.(From Wikipedia)

Zotero is a standalone program combined with a browser extension that lets you add online resources directly to your library. You probably can install it from your distro's sources.

Syncing can be done via a zotero online-account. Which is free but only entails a small bit of space.