zoteroZotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.| 论文文献管理工具
PDFdoPDFdo提供全套的PDF转换操作,一站式服务,在线转换PDF永久免费,可是实现各种PDF解密去除限制、各种PDF转换、PDF旋转页面、PDF删除页面、提取PDF页面、PDF拼接页面、PDF删除文字、PDF替换文字、PDF添加水印等功能。
SmallpdfSmallpdf - the platform that makes it super easy to convert and edit all your PDF files. Solving all your PDF problems in one place - and yes, free.