Academic search engines: such as Google Academic, Baidu Academic, China HowNet, Wanfang, etc. These academic search engines contain a large number of academic documents. You can find the papers you need by searching keywords and provide them for free download.

Academic social networks: for example, ResearchGate, academia, etc. These social networks can help scholars communicate and share their research results, and many papers can be downloaded for free.

Open access databases: such as arXiv, PLOS, PubMed? Central, etc These databases provide a large number of free and open access academic documents, many of which are research results uploaded by scientists themselves.

Academic journal websites: Some academic journals will provide some free papers for readers to download, such as ScienceDirect and SpringerLink, and readers can consult and download some open-access articles for free.