Merge PDF
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Merging copies every page of every file, in the order you set, into one new document. The pages themselves are untouched; same text, same fonts, same resolution, so the merged file is roughly the sum of the originals and nothing is re-compressed on the way through.
How to merge PDFs
What survives a merge is worth knowing, because most tools do not say. Text stays selectable and searchable. Internal links that point within their own file keep working. Bookmarks, form fields and digital signatures generally do not survive. A signature in particular is invalidated by definition, because it certifies a document that no longer exists. If any of that matters, merge first and add the signature to the finished file.
Questions
No. It runs on your own device, so the limit is your device’s memory. Hundreds of pages are routine.
No. Pages are copied intact, not re-rendered. The merged file is about as large as the originals added together.
Because nothing was compressed. If size matters, run the result through the PDF compressor afterwards.
Only if it is protected against editing rather than opening. A file that needs a password to open cannot be read at all.
Usually not. Merging builds a new document from the pages, and those structures belong to the old one.
No. The parser runs in this page. You can disconnect from the network and it still works.