I need to form fill documents, but EO.Pdf is not able to open any secured document. None of the documents require a password to view in Acrobat, they are only secured for modification. The EO.Pdf.PdfDocument constructor fails.

The file version of the EO.Pdf.dll is 5.0.69.2

My code is simply:
EO.Pdf.PdfDocument pdf = new EO.Pdf.PdfDocument("c:\\path\\to\\doc.pdf");

The first exception thrown, which is caught internally by EO.Pdf, is:
A first chance exception of type 'EO.Pdf.Internal.o' occurred in EO.Pdf.dll
Additional information: Header checksum illegal
Stack trace:
EO.Pdf.dll!EO.Pdf.Internal.ad.e() + 0x2af bytes
EO.Pdf.dll!EO.Pdf.Internal.ad.a() + 0x71e bytes
EO.Pdf.dll!EO.Pdf.Internal.ad.a(byte[] A_0, int A_1, int A_2) + 0x391 bytes
EO.Pdf.dll!EO.Pdf.Internal.ll.c(byte[] A_0, int A_1, int A_2) + 0x165 bytes
EO.Pdf.dll!EO.Pdf.Internal.jy.b(byte[] A_0) + 0x17f bytes
EO.Pdf.dll!EO.Pdf.Internal.jy.b(byte[] A_0, System.IO.Stream A_1, EO.Pdf.Internal.ai A_2, int A_3) + 0x35e bytes
EO.Pdf.dll!EO.Pdf.Internal.h4.b(byte[] A_0, string A_1, EO.Pdf.Internal.ai A_2, int A_3) + 0x89 bytes
EO.Pdf.dll!EO.Pdf.Internal.h4.d() + 0x1a5 bytes
EO.Pdf.dll!EO.Pdf.Internal.h4.a(EO.Pdf.Internal.h4[] A_0) + 0xdb bytes
EO.Pdf.dll!EO.Pdf.Internal.h4.e() + 0x6b bytes
EO.Pdf.dll!EO.Pdf.Internal.g4.a(EO.Pdf.Internal.hi A_0, EO.Pdf.Internal.h4 A_1) + 0x160 bytes
EO.Pdf.dll!EO.Pdf.Internal.g4.a(EO.Pdf.PdfDocumentSecurity A_0) + 0x89f bytes
EO.Pdf.dll!EO.Pdf.Internal.g4.a(bool A_0, EO.Pdf.PdfDocumentSecurity A_1) + 0x9a bytes
EO.Pdf.dll!EO.Pdf.Internal.hi.a(System.IO.Stream A_0, bool A_1) + 0xb9 bytes
EO.Pdf.dll!EO.Pdf.Internal.hi.a(string A_0) + 0x7f bytes
EO.Pdf.dll!EO.Pdf.Internal.hi.hi(string A_0, EO.Pdf.PdfDocumentSecurity A_1) + 0x160 bytes
EO.Pdf.dll!EO.Pdf.PdfDocument.PdfDocument(string fileName) + 0x3a bytes

EO then appears to have either handled or ignored the above exception, because the one actually thrown back to me is:
A first chance exception of type 'System.IO.InvalidDataException' occurred in System.dll
Additional information: Block length does not match with its complement.
Stack trace:
System.dll!System.IO.Compression.Inflater.DecodeUncompressedBlock(out bool end_of_block) + 0x184 bytes
System.dll!System.IO.Compression.Inflater.Decode() + 0x268 bytes
System.dll!System.IO.Compression.Inflater.Inflate(byte[] bytes, int offset, int length) + 0x97 bytes
System.dll!System.IO.Compression.DeflateStream.Read(byte[] array, int offset, int count) + 0x41 bytes
EO.Pdf.dll!EO.Pdf.Internal.jy.a(byte[] A_0) + 0x18a bytes
EO.Pdf.dll!EO.Pdf.Internal.jy.b(byte[] A_0, System.IO.Stream A_1, EO.Pdf.Internal.ai A_2, int A_3) + 0x387 bytes
EO.Pdf.dll!EO.Pdf.Internal.h4.b(byte[] A_0, string A_1, EO.Pdf.Internal.ai A_2, int A_3) + 0x89 bytes
EO.Pdf.dll!EO.Pdf.Internal.h4.d() + 0x1a5 bytes
EO.Pdf.dll!EO.Pdf.Internal.h4.a(EO.Pdf.Internal.h4[] A_0) + 0xdb bytes
EO.Pdf.dll!EO.Pdf.Internal.h4.e() + 0x6b bytes
EO.Pdf.dll!EO.Pdf.Internal.g4.a(EO.Pdf.Internal.hi A_0, EO.Pdf.Internal.h4 A_1) + 0x160 bytes
EO.Pdf.dll!EO.Pdf.Internal.g4.a(EO.Pdf.PdfDocumentSecurity A_0) + 0x89f bytes
EO.Pdf.dll!EO.Pdf.Internal.g4.a(bool A_0, EO.Pdf.PdfDocumentSecurity A_1) + 0x9a bytes
EO.Pdf.dll!EO.Pdf.Internal.hi.a(System.IO.Stream A_0, bool A_1) + 0xb9 bytes
EO.Pdf.dll!EO.Pdf.Internal.hi.a(string A_0) + 0x7f bytes
EO.Pdf.dll!EO.Pdf.Internal.hi.hi(string A_0, EO.Pdf.PdfDocumentSecurity A_1) + 0x160 bytes
EO.Pdf.dll!EO.Pdf.PdfDocument.PdfDocument(string fileName) + 0x3a bytes

If you need the PDF file, let me know how to provide it.

Again, I'm not expecting to modify the PDF content (we won't have the owner password), I just need it to open so I can form fill.

Many thanks.