Comment by pdfmergely

Our merge is page-level, not form-aware. We copy the pages (including the visual appearance of form fields), but we don't merge the PDFs' AcroForm dictionaries.

As a result, form fields typically aren't fillable after merging, and field name conflicts aren't an issue, so we don't rename fields.

We use @cantoo/pdf-lib (a maintained fork of pdf-lib) running entirely client-side in a Web Worker, so all processing happens locally in the browser and no files leave the user's device.