Merge a stack of PDFs in one pass

Twelve supplier invoices and a Friday filing deadline

It is month-end and the auditor wants one PDF with all twelve March invoices, in chronological order, before Friday. The files sit in a folder named march-invoices: 2026-03-invoice-01.pdf through 2026-03-invoice-12.pdf, 31 pages and 8.4 MB in total. Because the names sort chronologically, dropping the whole folder in lands them in the right order already, and the totals under the list confirm it: twelve files, 31 pages.

1. Open Merge PDF and select all twelve files in one go with Choose PDFs, or drag them in together.
2. Read the File order list top to bottom. If a file sits in the wrong slot, move it with its Move up and Move down arrows. Nothing re-sorts the list for you.
3. Press Merge PDFs. The browser joins the pages, then reopens the new file and compares its page count and selectable text with the twelve sources.
4. Read the proof line, press Download, and save the file. It arrives named after the first file with -merged added, so rename it to march-invoices.pdf before it goes to the auditor.

Well-named files make this boring, which is the point. The tool never sees the folder, only the files you hand it, so names like scan001.pdf through scan012.pdf would need a careful pass with the arrows first. Rename before you merge and the list checks itself.

One pass or pairs: what each costs you

Twelve files merged two at a time means eleven separate merges and ten intermediate files with names you invent on the spot. Each pass is a fresh chance to put a file in the wrong place, and none of the intermediate results gets the full twelve-source comparison. One pass means one list to check and one proof line to read.

Pairs have exactly one valid use: a set so large it exhausts the browser tab. Then two halves merged separately, and the halves merged after, is the documented workaround. Anything that fits in memory goes faster and safer in one pass.

Where batch merges go wrong

Trusting alphabetical order on bad names. A folder of scan001.pdf to scan012.pdf sorts fine until scan10.pdf lands between scan1.pdf and scan2.pdf. Rename the files with zero-padded numbers before you merge, or budget five minutes with the arrow buttons.

The same invoice twice. A duplicate hides easily in a long list. The page total gives it away: if twelve invoices should hold 31 pages and the list claims 34, scan the names for the repeated one and press its Remove button.

One locked file in the set. A password-protected PDF never joins the list; the tool refuses it up front. Nothing merges until every file is accepted, so open the locked file in Unlock PDF with the password you already know and add the unlocked copy.

Sideways pages inside the set. Each page keeps the orientation it had, so one landscape scan in a portrait set stays sideways in the output. Straighten the result afterwards with Rotate PDF, which can turn a single page without touching the rest.

The set outweighs the tab. Twelve text invoices are nothing; twelve 200-page scanned archives are a different animal. If the run fails on memory, merge in two halves and then merge the halves. Three passes instead of one, but each pass fits.

When a browser tab is not the right place

Most sets belong here, but four cases do not. A set of several hundred megabytes of scans can exceed what a tab holds; a desktop app with no tab ceiling handles it in one go. A file whose password nobody knows cannot be merged anywhere until the sender unlocks it or shares the password. A merge you repeat every week with the same filenames is a script job, not a browser job, and a desktop workflow will save you the monthly tedium. And a bundle whose bookmarks must survive needs a desktop tool that preserves document outlines, because a page copy may drop them.

If the merged set comes out too heavy for the auditor's inbox, run it through Compress PDF and type the size the mailbox allows. If you merged too much and need the result broken back up, Split PDF cuts it at the pages you choose. And if two files turn out to be all you ever had, the two-file page covers that simpler case.