Merging
Check page size and orientation across the source files before merging. Mixing A4 portrait with letter landscape produces a document that prints badly.
Decide the order deliberately and rename source files with a numeric prefix so the merge order is unambiguous.
Merging preserves each source page as it was; it does not re-render or degrade them.
Splitting
Split at logical boundaries such as chapters, invoices or cases, not at arbitrary page counts, so each file stands alone.
Name the outputs with content, not numbers. "invoice-2026-04-northwind.pdf" is retrievable a year later; "split_7.pdf" is not.
Compressing without ruining the file
Compression works by downsampling images and re-encoding them. Text-layer PDFs compress very little because they contain almost no image data; scanned PDFs compress dramatically.
Reduce colour depth before reducing resolution. Greyscale conversion is nearly free in quality terms for text documents.
Always compare a compressed page to the original at full zoom before deleting the source. Blur that is invisible on screen at 50 per cent becomes obvious in print.
Keep an original
Compression is not reversible. Archive the uncompressed file somewhere before you distribute the small one.
For records with legal or financial significance, the uncompressed original is the one to keep.