Rules and conventions for labeling documentary evidence -- I write “exhibit A” on the doc but it’s getting resistence

I often supply documents as evidence to regulators (e.g. GDPR regulators). A document is normally in A4 format and I digitally superimpose that onto an A4 page. Thus generally without shrinking or expanding.

I label it by printing “exhibit A”, “bewijsstuk A”, or “pièce A” in the topmost rightmost corner at a 45° angle and give a small margin to avoid unprintable areas. I do that on every single page. If it would overlap something, I shift it down to avoid overlap. It seems to do the job well but a regulator once requested that I resubmit the evidence without my markups.

So apparently they don’t like my style. Maybe they wonder if I could be making more material alterations. What is the normal convention in the legal industry? These evidence submissions are not for a court process but they always have potential to end up in court in the future.

I have some ideas:

• (only for paper submissions) I could stick a Post-It note to every document (every page?) and hand-write evidence labels. This would be inconvenient for them to scan. If they remove the notes to feed into a scanner, then the digital version is lossy and so they cannot dispense of the paper version. Or they must be diligent with entering the label into the file’s metadata or filename.
• (only for electronic submissions) I could make the evidence label a PDF annotation, so when viewing the doc and printing it the user can decide whether to show/print annotations. This seems useful superficially but it’s problematic because the PDF tools poorly adhere to the standard to w.r.t. annotations. Many tools do not handle annotations well. A recipient’s app does not necessarily give them control over whether annotations appear, and how they appear (different fonts chosen by different tools and if a tool does not have the source font it may simply ignore the annotation). The 45° angle that sets it apart and makes it pop-out better is apparently impossible with PDF annotations. And with little control over the font it might look good in one viewer but overlap in another.
• (versatile for both kinds of submissions) I could shrink the doc to ~90% of the original size, put a frame around it, and push it low on the page to leave space at the top for metadata like evidence labels. The the label is obviously not altering the original.
• (versatile for both kinds of submissions) I could add a cover page to each doc with the sole purpose of writing “exhibit A”. Seems good for digital submissions but I really don’t like the idea of bulking out my paper submissions. It would add €1 to the cost for every ten docs.
• (versatile for both kinds of submissions) Perhaps I could get away with rotating “exhibit A” 90° and finely printing it along the edge of the margin. This could even be combined with bullet 3 and maybe with less scaling (~95%).

Any other ideas?