It is quite likely that problem is already a known problem but I could not locate related bug.
I am using a latest android samsung phone . I am able to open djvu format pages from Special:IndexPages lists of en & mr.wikisource but unable to open pdf / PDF format pages.
for an example unable to open https://en.wikisource.org/wiki/Index:Book_of_common_prayer_(TEC,_1979).pdf on my mobile. But at the same time I can access individual pages of that file in 'page namespace if I choose access them from recent changes. But not the Index page.
Since mr.wikisource is largely having PDF format books and we are encouraging educational institutions to participate more in proof reading project. PDF pages not opening in Index namespace on mobile phone becomes too restrictive.