Putting together a packet
People usually combine PDFs to hand over one file instead of five: a job application with a CV, cover letter and certificates, an expense report with receipts, a contract with its schedules, or a set of scanned forms. The order matters more than anything else, because the reader goes through the file from the first page. Put the document that explains the rest first.
Check the pages in the preview before downloading. It shows every page of the combined file, so a missing page or an upside down scan is easy to spot, and you can go back with Edit files and fix it without starting again.
Scans, photos and exports together
Documents exported from a word processor contain real text, while a scan or a phone photo saved as PDF is only a picture. Combining them is fine, and each page stays what it was: the exported pages remain searchable, and the scanned pages remain pictures. ClipLime does not recognise text in pictures, so a page that was scanned will not be searchable in the result.
Scans are often turned sideways or come out at a different size from the rest. Pages of different sizes can share one file, and the rotate buttons in the page grid turn a single page left or right before the files are joined.
Before you send it
The combined file is about as big as the files you started with, because pages are copied rather than recompressed. If you plan to email it, look at the size shown after the merge before you attach it.
Bookmarks, links that jump inside a file, fillable form fields and the title and author of the originals do not carry over, so a combined form will look right but cannot be filled in. Password protected PDFs cannot be combined at all, because ClipLime cannot decrypt them. The page about merging PDFs explains these limits in more detail.