Attached PDF file(written in Japanese) is damaged after uploaded via VPN. How can I avoid this failure?

[kimuchundo]

We are currently working with an IBM workflow application called FormWave and running an electronic approval system. 
Currently we are working at home and using VPN((wifi at home). 
We are struggling with the investigation because there is a problem that the file is damaged when uploaded via VPN. 
Corruption means that when viewed with a binary editor, some data becomes zero bytes.

-Environment and Conditions
・App: IBM FormWave for WebSphere 6.1.2.3
・Middle: IBM WebSphere Application Server 7.0.0.13
・DB: IBM DB2 9.7.3
・OS: IBM AIX 6.1 TL7 SP6
・VPN: 2 channels(VPN1, VPN2) via wifi at home

 

*Not occur
・Internal network(company)
・Tethering(via VPN) from iPhone which is provided by company

-Steps
① Convert to PDF file written in Japanese on PC
② Save the PDF file as an attached file to the electronic approval that is connected by vpn
③ Download the file saved in electronic approval on your PC

 

-Problems
When opening the file attached to the electronic approval via VPN(wifi), the following events occur.
・Color unevenness of image data (ex. the background becomes greenish)
・Characters are faint
・Font cannot be read and characters are not displayed.
・Error displayed when opening a file
・occurs only at PDF files and other files like Excel, word are displayed correctly.
・different results between VPN1 and VPN2 and VPN1 occur less events.

 

We investgated VPN differeces between VPN1 and VPN2 because there are different results from both VPNs.
We found large sized files may occur events more frequently than small sized files.
There is regularity in the place where the replacement by zero occurs.
Although it is different from the replacement part, it has the common point that the size of the replaced data is 
always 5 bytes. There are cases where replacement occurs with less than 5 bytes 
but it occurs when the data before replacement was originally zero.
What could be the cause?

 

Any help would be appreciated.

 

-The first image is binary data comparison.
left is original and right is uploaded file. blue parts are turned to red parts(00 byte)