Chromebooks are excellent at most things and awkward at exactly this one. There is no Preview, no Acrobat, and the built-in Gallery app will show you a PDF but will not combine two of them. Search for a solution and you get a wall of Chrome extensions asking for permission to read everything you do.
The straightforward way
You do not need an extension. A browser-based merger works natively on ChromeOS because the work happens in the browser itself:
It lands in your Downloads folder like any other download. No extension, no install, no account.
Why this works when desktop software does not
ChromeOS runs web applications, so a tool that does its processing in JavaScript inside the browser is effectively native there — it does not need to be "ported". Merging PDFs is a structural operation: it copies page objects from several documents into one. That runs perfectly well in a browser tab.
It also means your files stay on your Chromebook. Nothing is uploaded, which matters more than usual on a school or work-managed device where you may not know what the network logs.
Reordering, and the file-picker trap
The one thing to watch: selecting several files at once in the ChromeOS file picker does not guarantee the order you expect. The picker returns files in its own order, which is often alphabetical rather than the order you clicked them.
Check the thumbnails after adding, and drag them into place before merging. This takes five seconds and saves redoing the whole thing.
If your files are named "Page1.pdf" through "Page12.pdf", be aware that alphabetical sorting puts "Page10" immediately after "Page1". Renaming to "Page01", "Page02" and so on fixes this permanently, and is worth doing if you handle these regularly.
Merging files from Google Drive
Files in Drive are not on your Chromebook until you open them. Two approaches:
Download first. Right-click in the Files app and choose Download, or make the file available offline. Then merge from Downloads.
Or drag directly. The ChromeOS Files app exposes Drive as a folder, and dragging from there into the browser generally works — ChromeOS fetches the file as part of the drag. If it fails, fall back to downloading first.
What merging keeps and what it drops
Straight answer, because most guides skip it:
Kept: all page content, text, images, page sizes and orientation. Pages that were landscape stay landscape. Text stays selectable and searchable.
Dropped: bookmarks and outline structure, form fields, and in most cases annotations and internal links between pages. If you are merging filled-in forms and you need the entered values, print each to a flattened PDF first — otherwise the field data may not survive.
For the common cases — combining scanned pages, assembling a set of receipts, joining chapters of a report — none of that matters.
Working entirely offline
If your Chromebook is offline, browser-based tools cannot load. Two fallbacks:
The Linux (Crostini) environment, if enabled, gives you "pdfunite" from the poppler-utils package — a single command that merges files and works with no network at all.
Or Android apps via the Play Store, if your Chromebook supports them. They install locally and run offline, though the quality varies wildly and many are aggressively ad-supported.