The Document Viewer application (also known as Evince) that serves as the default PDF viewer on many Linux distributions does a great job at displaying PDF documents. But if you need to manage a PDF file, such as merge, split, or rearrange the pages of the document, you will need another tool. There are many options, but the best for many cases will be PDF Arranger.
PDF Arranger has features that include many common aspects of PDF management and editing, including allowing you to merge documents, split a document, delete pages, rotate, and rearrange pages. If you need to manipulate a PDF document, this should be your go-to application and should meet the needs of most common PDF editing needs. The user interface is simple and straightforward to use, so you should be able to get your PDF files manipulated in the way you want in very little time.
PDF Arranger is free and open source, and can easily by installed from Flathub on any flatpak-enabled distribution such as elementary OS or Vanilla OS. If you need to edit a PDF document, don’t waste time weighing all the different tools and options, give PDF Arranger a go. There is a good chance it will help you accomplish what you need to do.