If you have some pdf files that you want to merge it, or one pdf files with many pages on it, then you can merge/split using pdf split/merge tools. But if you search on search engine "merge pdf", "split pdf", or "merge/split pdf", most of the results on search engine are tools that need internet connection or you must use it online. Is there offline solution, of course, yes, there are a good software and also this software free and open source. It's PDFSam.
For free version you can try PDFSam Basic.
PDFsam Basic, a free and opensource multiplatform software designed to split, merge, extract pages, mix and rotate PDF files.
Download it here:
User interface after installation:
On the top sections is free features that you can use for free, and for premium features, on the bottom.
To split PDF just choose SPLIT.
You can set the split settings, output files, filename settings, if done then choose/press Run button on the bottom.
To merge PDF just choose Merge:
Then drag and drop files ont the top:
On the top list is the first page, and the bottom list is the last page.
Set the merge settings, destination files, and filenames, then choose/press run button.
That's it, there are another free features such as split by bookmark, rotate, alternate, mix, extract, and split by size. You can try by yourself, if it is not enough then you can upgrade to premium versions.The premium versions have features such as Visual Combine & Reorder, visual split, crop, convert to excel, word, ppt, edit or customize pdf documents, and compress pdf file using PDFSam web service.