Application forms and claims nearly always want one attachment, not five. Joining the documents first is usually the difference between a submission that goes through and one that bounces.
Illustrative example: place an application first and evidence second. If only two evidence pages are required, extract those before merging rather than sending the complete evidence file.
Pages are copied without rasterising their content. Merging does not compress the resulting document.
Check document-level forms, bookmarks, links and signatures separately: rewriting a PDF does not guarantee they survive unchanged.
Each page keeps its own size and direction, so an A4 portrait document and a landscape one can sit in the same file. That is usually fine, but a page that came out sideways can be turned afterwards with Rotate & reorder.
Look at the page total shown after the merge and check it equals the two documents added together. If a file needs a password to open, remove it first with Protect PDF, because a locked document cannot be merged.
No. It combines pages. Raster compression is a separate operation and removes searchable text.
A file that needs a password to open must be unlocked first with Protect PDF. If a very large merge runs out of memory, join the files in two stages and then join the two results.
There is no set limit. The practical limit is your device's memory, so for a very large job, merge in two stages.