Decide the reading order first
Put a cover page or application form first, followed by the evidence in the order referenced by that form. A typical packet might be: application, identity document, certificate, then supporting records.
Use filenames as a temporary checklist
If the source files are hard to distinguish, rename local copies with simple prefixes such as 01-cover, 02-form, and 03-certificate. The merged PDF does not need those prefixes; they only help you assemble it.
Check orientation and page size
A merged file can contain portrait, landscape, A4, and Letter pages. That is valid, but abrupt changes can be confusing. Rotate pages in the source application first if their visible orientation is wrong.
Keep the original files
Merging creates a new PDF. It can remove document-level features such as bookmarks, forms, attachments, accessibility structure, or digital signatures. Preserve every original until the submission is accepted.
Inspect the finished packet
Open the download, scroll through every page, confirm the first and last pages, and compare the total page count with the sum of the originals. Check the final upload limit before submitting.