There exists a known limitation when opening a PDF URL within the IOS Salesforce Mobile Application.
Note:
The current limitation has been observed at the publication time of this article; subsequent IOS and Salesforce Mobile Application versions may present a dissimilar behavior from that presented here.
For example, the screenshot below illustrates a Visualforce Page that is displaying:
A SharinPix Album
A hyperlink containing a URL (towards a PDF file) in the form http://www.pdf995.com/samples/pdf.pdf. Upon clicking on the hyperlink, you will be directed to a view as shown below.
Limitation
LIMITATION: The above view does not offer an option to save the current PDF file into a storage medium.