For those who use Telegram, there is a Telegram bot, where you send it the DOI or the URL and it sends you the PDF -- so much easier than legally accessing the file.
The site returns direct pdf links only for paywalled journals. If you put in a DOI for an article that's hosted on a free-access journal, the site redirects you to the article's page on the journal, so you can download the pdf directly from them. This behavior might be the cause?