The PDF document linked in the documentation home does not contain full docs, but rather what is displayed on this very web page.
https://docs.spring.io/spring-framework/docs/5.3.20/reference/pdf/index.pdf
Looks like it is related to #26722. Single page HTML documentation is not available, similar with PDF file.
Each chapter is exported to a separate PDF:
https://docs.spring.io/spring-framework/docs/current/reference/pdf/
Either PDF containing complete documentation should be generated and linked on the documentation home page, or the link should be removed.