Hi Chloe,
How is your pdf file shown on your web page? We have multiple landing pages that our team uses and I have a Samsung Galaxy Note 3 that I use to test our landing pages and forms. When a user fills out a form, they have the option to read a PDF via a link and when that is clicked on in my Samsung phone browser, they have the option to read the PDF either within my Adobe app or the Chrome/Internet app.