How to merge PDF files into one: what we measured about size, duplicates and fillable forms

Merging PDFs sounds like the simplest job there is: put the pages of several files in one file. It mostly is, and most guides stop there. We merged a mix of test files with the same library our Merge PDF tool uses and measured what comes out, and found three things worth knowing before you merge something important: the size of the result, what happens to duplicates, and what happens to a fillable form.

How the Merge PDF tool works

You add two or more files, put them in the order you want with the move up and move down buttons, and the Merge PDF tool copies every page of every file, in that order, into a new document. It runs in your browser, so the files are not uploaded. Because it copies pages as they are, text stays text: it remains selectable and searchable, and nothing is turned into pictures the way compression does.

How big will the merged file be?

We merged four combinations and compared the result with the sum of the inputs:

For image-heavy files the merged size equals the total, give or take a few hundred bytes. The only real saving was among the tiny text files (23%), because they shared a little internal overhead, and it amounts to under 2 KB. So if a portal limits you to 200 KB, the merge will not help you; the parts must already add up to under it.

Duplicates are kept in full

The last row is the one to notice. Three copies of the same photo PDF produced three copies of the same pictures inside the merged file. Nothing noticed they were identical. If you are assembling an application and the same certificate appears in two of your source files, it will be stored twice. Open each file first and drop the repeated pages with Delete PDF Pages or pull out only the ones you need with Extract Pages.

What happens to a fillable form

We merged our sample registration form (one page, 10 form fields: text boxes, a drop-down, radio buttons and check boxes) with a two-page report.

The boxes are still drawn on the page, but the merged document no longer lists any form fields. In practice you should expect it to stop behaving like a fillable form, and we have not tested how each PDF viewer treats the leftover boxes, so check yours. The safe rules are simple. If people still need to type into the form, send it as a separate file. If you only need a filled-in copy, fill it first with the PDF Form Filler, merge, and then look at every page of the result before you send it.

The order that avoids rework

1. Fix each file first. Rotate sideways pages with Rotate PDF, remove blank or repeated pages, and reorder pages inside a file with Reorder PDF Pages.
2. Name the files so they sort in the order you want. 01-cover, 02-statement, 03-certificate makes the sequence obvious when you add them.
3. Add them to the Merge PDF tool and check the order using the move up and move down buttons.
4. Merge, then open the result and page through it. Confirm the page count equals the sum of the files and that every page is the right way up.
5. If it is too large, shrink the picture-based parts, not the whole file. Compression turns pages into images, so a text page gets bigger when you compress it. Compress only the scans, then merge. Our PDF compression guide shows what to expect, and Split PDF undoes a merge if you need the parts back.

What we did not test

We did not test bookmarks, internal links, page labels, tagged or accessible PDFs, or password-protected files, and we did not check the results in every PDF viewer. The tool opens files with their encryption flag ignored, so if a protected file looks wrong after merging, remove the password first with the PDF Unlocker. We used a handful of small test files, so very large files may take longer or run into your browser's memory limit.