static void Main(string[] args)
{
    PdfCommon.Initialize();
    IntPtr docHandle = Pdfium.FPDF_LoadDocument(@"C:\FillableFields.pdf", null);
    if (docHandle == IntPtr.Zero)
        throw ProcessLastError();
    using (var doc = PdfDocument.FromHandle(docHandle))
    {
        //Gets second page from document;
        PdfPage page = doc.Pages[1];
        //Gets number of characters in a page or -1 for error.
        //Generated characters, like additional space characters, new line characters, are also counted.
        int totalCharCount = page.Text.CountChars;
        //Extract text string from the page
        string text = page.Text.GetText(0, totalCharCount);
    }
}

public static Exception ProcessLastError()
{
    uint code = Pdfium.FPDF_GetLastError();
    switch (code)
    {
        case 1: return new UnknownErrorException();
        case 2: return new PdfFileNotFoundException();
        case 3: return new BadFormatException();
        case 4: return new InvalidPasswordException();
        case 5: return new UnsupportedSecuritySchemeException();
        case 6: return new PageNotFoundException();
        case 126: return new PageNotFoundException();
        case 0x20000000 | 0x00001: return new ImageObjectIsEmptyException();
        case 0x20000000 | 0x00002: return new NoLicenseException();
        case 0x20000000 | 0x00003: return new NoLicenseException("The requested operation cannot be completed due to a license restrictions. You need to have at least a Developer Small Business License to be able  to complete this operation.");
        case 0x20000000 | 0x00004: return new NoLicenseException("The requested operation cannot be completed due to a license restrictions. You need to have at least a LITE License to be able  to complete this operation.");
        case 0x20000000 | 0x00005: return new NoLicenseException("The requested operation cannot be completed due to a license restrictions. You need to have a  full license to be able  to complete this operation.");
        case 0x20000000 | 0x00011: return new PdfiumException(code, "Can't find parent dictionary for given bookmark.");
        default: return new PdfiumException(code, "Unexpected error code.");
    }
}