added binary comment as attribute and for load and write. Binary Comment is gonna be important for pdf in A/2, A/3 format.

[aedn-sama]

No description provided.

[aedn-sama]

Checked it against veraPDF.
Second line should be the binary marker.

// Load the existing PDF file
let mut pdf = Document::load("537_INPUT.pdf")?;
// let mut pdf = Document::load("output-pdfa3.pdf")?;
pdf.delete_zero_length_streams();
    
// pdf.binary_comment = vec![0xFF, 0xFF, 0xFF, 0xFF];
pdf.binary_comment = vec![0xC1, 0xC2, 0xC3, 0xFF];

Can be manipulated over public attribute.
If binary_comment is being untouched and the loaded document has a binary marker itself, it will get taken over.

[Heinenen]

Looks good to me.
FYI, the normal PDF spec demands that a binary comment is written (it's only strictly necessary if there is binary data anywhere else in the file, but we wouldn't want to check for that anyway).

A downside of your implementation (for some users) is the immediate rejection of the file if it doesn't contain the binary comment.
Although this is spec-compliant, some users may still complain, cf. #41.