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

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, custom, partial, or location-based assembly bind).
LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework/v4.0.30319/Temporary ASP.NET Files/root/ccfa215a/dc93c54d/PDFNet64.DLL.
LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework/v4.0.30319/Temporary ASP.NET Files/root/ccfa215a/dc93c54d/PDFNet64/PDFNet64.DLL.
LOG: Attempting download of new URL file:///c:/users/mirko.lugano/documents/visual studio 2012/Projects/WebApplication3/WebApplication3/bin/PDFNet64.DLL.
ERR: Failed to complete setup of assembly (hr = 0x8007000b). Probing terminated.


Stack Trace:


[BadImageFormatException: Could not load file or assembly 'PDFNet64' or one of its dependencies. An attempt was made to load a program with an incorrect format.]
   System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) +0
   System.Reflection.RuntimeAssembly.nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) +34
   System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, RuntimeAssembly reqAssembly, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) +152
   System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean forIntrospection) +77
   System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection) +16
   System.Reflection.Assembly.Load(String assemblyString) +28
   System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) +38

[ConfigurationErrorsException: Could not load file or assembly 'PDFNet64' or one of its dependencies. An attempt was made to load a program with an incorrect format.]
   System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) +752
   System.Web.Configuration.CompilationSection.LoadAllAssembliesFromAppDomainBinDirectory() +218
   System.Web.Configuration.CompilationSection.LoadAssembly(AssemblyInfo ai) +130
   System.Web.Compilation.BuildManager.GetReferencedAssemblies(CompilationSection compConfig) +170
   System.Web.Compilation.WebDirectoryBatchCompiler..ctor(VirtualDirectory vdir) +124
   System.Web.Compilation.BuildManager.BatchCompileWebDirectoryInternal(VirtualDirectory vdir, Boolean ignoreErrors) +44
   System.Web.Compilation.BuildManager.BatchCompileWebDirectory(VirtualDirectory vdir, VirtualPath virtualDir, Boolean ignoreErrors) +187
   System.Web.Compilation.BuildManager.CompileWebFile(VirtualPath virtualPath) +74
   System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate) +299
   System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate) +103
   System.Web.Compilation.BuildManager.GetVirtualPathObjectFactory(VirtualPath virtualPath, HttpContext context, Boolean allowCrossApp, Boolean throwIfNotFound) +165
   System.Web.Compilation.BuildManager.CreateInstanceFromVirtualPath(VirtualPath virtualPath, Type requiredBaseType, HttpContext context, Boolean allowCrossApp) +43
   System.Web.UI.PageHandlerFactory.GetHandlerHelper(HttpContext context, String requestType, VirtualPath virtualPath, String physicalPath) +31
   System.Web.UI.PageHandlerFactory.GetHandler(HttpContext context, String requestType, String virtualPath, String path) +37
   System.Web.MaterializeHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +346
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155


Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.18033 

I have already searched a lot in the web and in google groups, and I have already tried these solutions (among the others), THIS and for instance THIS

I'm running a Windows 8 x64 PC, and I DO have the Visual c++ 2010 x64 redistributable installed on my machine. I have also tried the console applications provided by the samples downloaded from pdftron.com, and those work ok in all versions of .NET. I have tried creating a console app of my own and that too works on all versions of .NET. I had then thought it was maybe the fault of VS2012 development server which runs in x86 mode, so I set up a IIS website and run it from there, but still the same problem. I am a bit bereft, does anybody have any ideas? Thanx.

Issue was solved thanks to the support of the guys @ pdftron.com. Here is the solution. In my particular case, I had the 'Enable 32-bit applications' Application Pool setting inside IIS settings set to True, which somehow conflicted with the x64 version of PDFNet.dll. Resetting it to false solved the issue.