Merging PDFs looks like the most trivial operation there is, pick some files, get one file. And it mostly is. But a few small habits turn merging from "usually fine" into "reliable every single time." Here is everything worth knowing.
Order before you merge, not after
The most common merge regret is combining files in the wrong sequence and only noticing afterwards. Decide the order first, arrange your files, then merge. Fixing the order up front is far easier than reshuffling pages in the combined document later.
Mind the mismatches
Files from different sources often disagree about page size and orientation, an A4 report, a US-letter form, a landscape scan. Merging them is perfectly fine, but glance through the result so a sideways or oddly sized page does not surprise your reader. Rotating a stray page takes seconds once you have spotted it.
Clean the parts first
Merging faithfully preserves whatever is in each file, including the blank pages and duplicates. Removing the junk before combining is easier than hunting for it in a long merged document afterwards. Structure first, merge second.
Name the result well
A merged file called "merged.pdf" is a mystery in a week. Give the combined document a clear, descriptive name that says what it contains, so future-you does not have to open it to find out.
Keep it local for sensitive files
Merging often involves exactly the documents you would least like to upload, contracts, IDs, financial records assembled into one package. Combining them in your browser, where the files never leave your device, is the privacy-respecting way to do it.
