How can I access files on my network from my iPhone? I am attempting to access a PDF file on my network from my iPhone and I know how to access these files from Windows OS.

However since I have an iPhone 5, I don't have the knowledge to make that device work with my existing network files. 

I have attempted several apps and none seem to work. I am not interested in changing my network to support iOS. My intent is to absolutely have something on the iPhone to access the same sort of files I can from my Windows laptop when connected via VPN.
I recommend FileExplorer Free.  I just installed it to access files that I wrote on a SD card in my WiFi router.  I have an iPhone and iPad.  FileExplorer can use CIFS, SMB, FTP and others.  It can read many formats such as PDF, video, photos, music and Microsoft Office.
There is  priced version for five dollars.