A trove of leaked documents from a Chinese state-linked hacking group shows that Beijing’s intelligence and military groups are attempting large-scale, systematic cyber intrusions against foreign governments, companies and infrastructure — with hackers of one company claiming to be able to target users of Microsoft, Apple and Google. The cache — containing more than 570 files, images and chat logs — offers an unprecedented look inside the operations of one of the firms that Chinese government agencies hire for on-demand, mass data-collecting operations. The files — posted to GitHub last week and deemed credible by cybersecurity experts, although the source...