VS2017 ver 15.9.3

NuGet Packages PdfiumViewer version="2.13.0.0"

PdfiumViewer.Native.x86_64.v8-xfa version="2018.4.8.256"

Test Scenario - Load PDF file with PdfiumViewer

1. Create Project1: Target Framework = .Net Framework 4. Program runs successfully.
2. Change Project1 to any higher Framework - 4.5, 4.5, 4.6 ... 4.72 Program runs successfully

3. Create Project2: Target Framework = .Net Framework 4.7.2 or any other FW 4.5, 4.5, 4.6 ( ** NOT FW 4 ).
Error Message: $exception {"An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B)"} System.BadImageFormatException

4. Change Project2 to FW = 4. Program runs successfully.
5. Change Project2 to any higher Framework - 4.5, 4.5, 4.6 ... 4.72 Program runs successfully

Code

Error Message: $exception {"An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B)"} System.BadImageFormatException

  private void button1_Click(object sender, EventArgs e)
  {
     string pdf = @"D:\PDF\TestFile.pdf";

     try
     {
        var document = PdfiumViewer.PdfDocument.Load(pdf);   // *** Exception  ***
        MessageBox.Show("OK");
     }
     catch (Exception ex)
     {
        MessageBox.Show(ex.Message);
     }
  }

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Help is appreciated.

Add both the x86 and x64 folders containing pdfium.dll to the Debug/Release directories, that should be the way. In case you don't know where to find them, run the PdfiumViewer.Demo project from PdfiumViewer Github, you will find them inside the bin folder.