I use a low-medium range smartphone. I don't understand why when I log on to CTAN and want to download a pdf this doesn't happen. What is the reason?
When viewing content on CTAN, linking to files is done through mirroring, which poins to some readily-available server for the actual content. This mirroring may target servers that don't respond fast enough, possibly dropping the query. One way around it is to use a dedicated server. In the case of documentation (or PDFs), this is provided by
<pkg> represents the package you're interested in). The server finds the closest match to
<pkg>. For those packages with multiple documentation versions (which might be in different formats, or separate documentation for code and user interfaces, say), you're still forced to go the CTAN route.