Merging and splitting are opposite operations, but both solve the same practical problem: creating a document whose boundaries match the way it will be read, stored or shared.
Merge several PDFs into one file
Use Merge PDF files when separate files belong to the same dossier: chapters, invoices, supporting documents or scans. Arrange them in the intended reading order before generating the result.
Keep the source files until you have checked that every document is present and that the sequence is correct.
Split one PDF into several files
Use Split a PDF when a large document needs to become several independent files. A 30-page report, for example, can become three ten-page sections.
Splitting is especially useful when recipients should receive different sections or when a large archive needs clearer boundaries.
Split or extract?
If you need pages 1–10, 11–20 and 21–30 as three files, split the document. If you only need pages 3, 8 and 22, extract those pages instead.