How to Merge PDF Files: Methods, Pitfalls and Tips

Deze inhoud is in het Engels.

Combining several PDFs into one document is one of the most common everyday file tasks, and it is also one where small mistakes cause big headaches. This guide covers the main ways to merge PDFs, the pitfalls that catch people out, and how to keep the final file a sensible size.

Door Valtrinity. Bijgewerkt op 18 oktober 2026.

When merging PDFs is the right move

Merging makes sense when several documents belong together as a single record, such as a job application made up of a CV, a cover letter and certificates, or a set of scanned receipts for an expense claim. A single file is easier to email, upload to a portal and archive. Many online forms accept only one attachment, so combining files saves the recipient from hunting through separate downloads.

Merging is less useful when recipients need to edit each document independently, or when the documents have different owners and sign-off stages. In those cases a named bundle or a zipped folder may serve better. Think about who will open the file and what they will do with it before you combine anything.

Option one: use an online merge tool

The quickest route is an online merger such as the pdf-merge tool on this site. Upload your files, drag them into the order you want, and download the combined result. This suits most everyday jobs because there is nothing to install, and a handful of documents takes less than a minute.

Check upload limits before you start, especially for large scanned files. If the combined output is too large for your email or the portal, reduce it afterwards or split the bundle into logical parts. Pages that are mainly photographs respond well to compression, while text-only pages are usually small already.

Option two: desktop and command-line tools

Mac users can open several PDFs in Preview, show the thumbnail sidebar, drag pages into position and export the result. Adobe Acrobat includes a Combine Files function, and Windows users can often merge files through whichever PDF editor they already use. Choose the application you are most comfortable with, then test it on a copy first.

Command-line users can rely on tools such as pdfunite from the Poppler package or qpdf. For example, pdfunite first.pdf second.pdf merged.pdf joins two files in that order. These tools are dependable and easy to script for batch work, but they assume you are comfortable typing commands in a terminal.

Getting the page order right

Most merge problems come from order. Every tool processes files in the sequence you give them, so check the file list or thumbnails before you press the merge button. Filenames sort alphabetically, which means scan1, scan10 and scan2 will place page 10 before page 2 unless you pad the numbers with leading zeros, such as scan01, scan02 and scan10.

• Name files with leading zeros, such as part-01.pdf and part-02.pdf
• Check the first and last pages of each source before merging
• Remove blank or duplicate pages with pdf-split before you combine

Pitfalls that catch people out

Password-protected PDFs will not merge until you unlock them, and many tools refuse them outright. Scanned documents are images rather than text, which is fine for reading but makes searching impossible unless you run optical character recognition, or OCR, first. Merging also does not fix mixed page sizes, so an A4 page beside a US Letter page will still look uneven.

• Digital signatures are usually invalidated once a file changes, so sign after merging
• Bookmarks, hyperlinks and form fields can be lost or flattened by some tools
• Pages scanned at different rotations may need turning individually before combining

Managing file size

A merged file is roughly the sum of its parts, but it can grow more than expected when each source embeds its own copy of the same fonts or images. Several high-resolution scans can easily produce a file of 50 MB or more. Many email services cap attachments at around 20 to 25 MB, so check the total before sending.

To reduce size, start with the source. Scanning text documents at around 150 to 200 dpi is usually enough for on-screen reading, and it keeps files far smaller than very high settings. If your pages are mostly photographs, compress the original image files with the image-compressor tool and convert them with images-to-pdf before merging. Splitting a large bundle into parts is often kinder to recipients than squeezing it until it looks poor.

Adding page numbers after merging

Merging does not renumber pages. If each source file was numbered from 1, the combined document will show repeated numbers. The cleanest approach is to merge first, then add page numbers to the finished file so they run continuously from start to finish. Many PDF editors offer a page numbering option, which is useful for reports and legal bundles.

If numbers must match the source files, agree a convention before merging, such as a cover sheet listing each document and its page range. Numbers printed on the page itself are more reliable than PDF page labels, because some viewers ignore labels entirely.

A checklist before you share

Once the merged file is ready, run through a short check before it leaves your hands. Scroll through every page once, open the result in a different app to confirm it renders, and compare the page count with the total of your sources. Then save the final file with a descriptive name such as surname-application-2026.pdf, so the recipient can tell what it contains at a glance.

• Scroll through every page once to confirm the order
• Check the total file size against the recipient's limit
• Test links and bookmarks if they matter to the reader
• Keep the originals in case you need to split or edit again

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