How to Merge Two PDF Files Without Paying for a PDF Editor

TL;DR: To merge two PDF files, use any PDF viewer that supports page organisation — Apple Preview, LibreOffice Draw, or a browser-based merge tool. The operation is to open one file, insert or append the pages of the second, then save as a new document. Two things cause trouble: dissimilar page sizes produce a document with mixed dimensions, and encrypted files refuse to merge until the owner password is supplied.

The simplest reliable routes

Four methods cover almost every situation, and they differ mainly in where they run.

Apple Preview. Open the first PDF, then drag the second file’s pages into the sidebar, or use View → Thumbnails and drag between windows. Save immediately as a new file. This is the fastest method available and it costs nothing.

Browser-based merge tools. Upload both files, order them, download the result. They work on any platform and require no installation. The trade-off is uploading documents to a third party, which matters for anything confidential.

LibreOffice Draw. Open the first file, then Insert → Page from File to append the second. Save as PDF. Free, offline, and it handles page ordering well.

Command line, for scripted work. On Linux or macOS, qpdf --empty --pages a.pdf b.pdf -- out.pdf or pdfunite a.pdf b.pdf out.pdf merges without an interface. This is the right choice when the same operation has to run weekly.

None of these are worse than a paid editor for this task. Merging is a page-organisation operation, not a text-editing one, and the format makes it straightforward.

Ordering, not just combining

The common mistake is to assume merge means “append.” Usually what is wanted is a specific order, and every tool above supports dragging pages into position after the initial merge.

The practical approach is to merge everything first, then reorder by dragging thumbnails. Trying to place pages correctly while importing is more error-prone, and the visual thumbnail view makes the correct order obvious.

If you are assembling a document from several parts — cover, body, appendix, signature page — merge in reverse importance order, then rearrange. It sounds backwards and it reduces the number of drag operations.

Mismatched page sizes

This is the most visible artefact of merging, and it is worth understanding rather than being surprised by.

A PDF page has a defined physical size. Merge a US Letter document with an A4 one and the result contains both sizes; the viewer scales each page to fit the window, so it looks inconsistent when scrolled continuously and prints at two different dimensions.

Three options. Accept it, which is fine for internal use. Scale pages to a common size during merge, which most tools can do but which adds margins or clips content. Or convert the minority pages first — printing to PDF at the target size is the simplest way to normalise a single document.

The same applies to orientation. A landscape page inside a portrait document is not an error, but it will look wrong to a reader who is not expecting it, and rotated pages are worth fixing before merge rather than after.

The failure cases

Encrypted or permission-restricted files. If a PDF has an owner password that restricts editing, merge tools will refuse or silently skip it. The fix requires the owner password, and stripping restrictions without it is both technically awkward and legally questionable depending on the document.

PDFs with form fields. Field names must be unique. Merging two documents that both contain a field called “Name” can cause the values to collide, so the recipient’s answers appear in the wrong place. Flattening the forms to static content before merging avoids this.

Bookmarks and outlines. Merge tools vary in whether they preserve the outline. Many flatten everything into one level or drop bookmarks entirely, which matters for long documents where navigation was the point.

File size. Merging does not recompress. Two 20 MB scans produce a 40 MB file, and if the source used different image encodings the result can be larger than the sum. Compressing afterwards is usually necessary before emailing.

Compressing after the merge

The merged file is usually bigger than people expect, and the reason is that each source document’s images are carried over at their original quality.

Options that work without paying: most merge tools offer a “compress output” checkbox, which downsamples images. For a more controlled result, export the merged file through a tool that lets you set image resolution — 150 DPI is fine for screen reading and dramatically smaller than 300 DPI.

If the purpose is email or upload, check the destination’s size limit first. Many portals cap at 10 or 25 MB, and knowing the cap determines how aggressive the compression needs to be.

When merging is the wrong operation

Two cases are worth flagging because they waste time.

If you want a single file that looks continuous, merging is right. If you want to send two documents together for convenience, a ZIP or two attachments is simpler and preserves each file’s integrity.

If the pages will later be split apart again, keep the sources. Merging and re-splitting degrades nothing technically, but it loses the original file names and metadata, which is exactly what you need when separating them later.

And if the goal is to share a page as an image rather than as a document, merging is unnecessary work — exporting PDF pages as JPG or PNG gets a page into a chat window or a listing without any page assembly at all.

The short version

Merge with Preview, a browser tool, LibreOffice Draw, or a command-line utility — all free and adequate. Merge first, then reorder by thumbnails. Expect mixed page sizes, refused encrypted files, colliding form fields, and a file that needs compressing before it can be sent.