Answer: No. WikiLetters is not a PDF repository. WikiLetters brings you visual-links which can also be found via Google Scholar, Bing, Baidu, and even in Wikipedia articles such as,

www.wikipedia.org/wiki/PubPeer

www.wikipedia.org/wiki/Retraction_Watch

www.wikipedia.org/wiki/Sci-Hub

and others

Answer: The umbrella of WikiLetters highlights the source of relevant information supporting answers to key-questions concerning quality in science.

Answer: Databases under the umbrella of WikiLetters are complementary. WikiLetters joins these databases to support the better assessment of scientific publications and journals from different angles.

Answer: No. WL is a free to publish database relying upon the goodwill of scientists and researchers who recognise the need for better quality assurance in science and higher level of transparency.

Answer: Yes. Otherwise it would not be a Wiki-system.

Answer: Yes. WL provides a space for responses at the bottom in the very last section called "Comments".

Answer: Similarly to Letters, Notes aims to easily provide you with information about caveats and critiques to articles. However, this databases provides a more concise information, which has not yet reached the level required for a Letter.

Answer: Articles which are largely cited, may have received some caveats or critiques upon. WL-Comments is a resource to easily provide you only critiques to articles you aim to read and refer to into your own article.

Answer: WL-Retracted aims to easily provide you with information about retracted articles. Consequently, this information might help to reduce post-citation retractions.

Answer: Citing-retracted aims to easily provide you with information about articles that have cited retracted articles. Because you may wish to be cautious when using information from articles that have cited retractions.

Answer: Classified-citations aims to provide you with "subtle caveats or critiques" and "positive or relevant citations" that might be as important as full comments on papers you aim to read and refer to into your own article.

Answer: This database aims to inform you about minor corrections at the level of an Erratum and Corrigendum. Additionaly, to correct citations that have been found improper (i.e. incorrect) and sourced from an article you aim to read.