I'm trying to build a sample web application from a blank web form template in VS2012, and add the PDFNet dll to it. If I run the project in .NET 3.5 no problem. If I run it in .NET >= 4 I get this error:

Could not load file or assembly 'PDFNet64' or one of its dependencies. An attempt was made to load a program with an incorrect format.

this is the stack trace: 

Assembly manager loaded from:  C:\Windows\Microsoft.NET\Framework\v4.0.30319\clr.dll
Running under executable  C:\Program Files (x86)\IIS Express\iisexpress.exe
--- A detailed error log follows. 

=== Pre-bind state information ===
LOG: User = COLLAB\mirko.lugano
LOG: DisplayName = PDFNet64
 (Partial)
WRN: Partial binding information was supplied for an assembly:
WRN: Assembly Name: PDFNet64 | Domain ID: 2
WRN: A partial bind occurs when only part of the assembly display name is provided.
WRN: This might result in the binder loading an incorrect assembly.
WRN: It is recommended to provide a fully specified textual identity for the assembly,
WRN: that consists of the simple name, version, culture, and public key token.
WRN: See whitepaper http://go.microsoft.com/fwlink/?LinkId=109270 for more information and common solutions to this issue.
LOG: Appbase = file:///c:/users/mirko.lugano/documents/visual studio 2012/Projects/WebApplication3/WebApplication3/
LOG: Initial PrivatePath = c:\users\mirko.lugano\documents\visual studio 2012\Projects\WebApplication3\WebApplication3\bin
Calling assembly : (Unknown).
===
LOG: This bind starts in default load context.
LOG: Using application configuration file: c:\users\mirko.lugano\documents\visual studio 2012\Projects\WebApplication3\WebApplication3\web.config
LOG: Using host configuration file: C:\Users\mirko.lugano\Documents\IISExpress\config\aspnet.config
LOG: Using machine configuration file from C:\Windows\Microsoft.NET\Framework\v4.0.30319\config\machine.config.
LOG: Policy not being applied to reference at this time (private, cus
.net load .net-assembly
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