Prepare the files before merging
Give the source files clear names and open each one briefly to confirm it is the correct version. If you have many documents, numbering them 01, 02, 03 and so on makes the intended order easier to verify.
Arrange pages before creating the final file
A merge tool should let you control the order of the uploaded documents. Put cover pages first, supporting documents next and appendices last. For mixed documents, it is often faster to organize whole PDFs first and reorganize individual pages only if necessary.
Check the final PDF
After merging, open the result and move through the document from beginning to end. Confirm there are no duplicate pages, missing sections or blank pages. If the PDF contains bookmarks or forms, verify those features separately because some simple merge workflows may not preserve every advanced property.
Keep the originals
Do not delete the individual source files immediately. Keeping the originals makes it easier to rebuild the combined document if a section needs to be replaced later.
Final check
Online tools can save time, but the best result still comes from checking the output before you rely on it. Keep an original copy of important files and review any detail that matters to your work.