Merge PDF issue

Hi.

We merge pdf documents using such code:

var outputDocument = new Document();

foreach (var documentName in files)
{
       var inputDocument = new Document(documentName.Key);
       outputDocument.Pages.Add(inputDocument.Pages);
}
outputDocument.Convert(new MemoryStream(), PdfFormat.PDF_A_1B, ConvertErrorAction.Delete);
outputDocument.Save(resultFileName);

And faced with one document wich cannot be merged in such way (attached - Original.pdf).
If we convert it to pdf/a all is working fine (attached - Converted.pdf) but if we try to merge we get document with error in it (attached - Merged.pdf).

I have an assumption that forms in file causes this issue. Is it a Aspose bug or we may fix it in code?

Thanks