Our Merge Attachment Fields can do this for you in bulk.
Setup
Create a separate attachment field in Airtable for the combined result.
Create this automation and select the Airtable base and table you want to use.
Under Attachment Fields, select the attachment fields you want to combine. Lookup fields that contain attachments can also be selected.
Set Merged Attachment Field to the destination attachment field.
Review Processing Behavior if the output field already contains values. Only choose to overwrite data you intend to replace.
In Run, limit processing to a view containing one or two test records. Run the automation and check that the destination contains the expected files before processing more records.
This combines files into one attachment field; it does not merge the contents of PDF files into a single PDF. For that, use Merge PDFs. If you need filename-based duplicate removal from a lookup field instead, use Gather Unique Attachments.