Combining PDFs, and why it is switched off

Quick answer

Combine merges two or more staged PDFs into one row, and it is off by default because what the merged file is like depends entirely on what your server has installed.

Switch it on under Bulk Replace → Settings. The same screen lists what your server can actually do, which is the thing worth reading before you turn it on.

Five methods, and they are not equivalent

Four of the five combine the page objects: the text stays text, fonts stay embedded, and the file stays roughly the size of its parts added together. One of them does not. The PHP Imagick extension hands each page to ImageMagick, gets a picture back, and wraps the pictures in a PDF — so the output has no selectable text, nothing to search, nothing a screen reader can read aloud, and line art becomes pixels.

Measured on two one-page text documents: the image-based output had no extractable text, no embedded fonts and came to 190,854 bytes. A structural merge of the same two files kept every word and came to 2,908 bytes.

So the methods are tried most-faithful first, and the one that rasterises is tried last — where it is a better answer than refusing, but only just. When it runs, the result says so.

What your server probably has

• Bundled library. Ships with the plugin, so it is always available. Its free parser cannot read compressed object streams, which most current software produces, so it often declines a modern PDF.
• pdftk and Ghostscript. Command-line tools, both excellent, neither installed by default on most shared hosting. Ghostscript is usually the easiest thing to ask a host for, and it keeps the text.
• Imagick. Three separate questions — whether the PHP extension is loaded, whether ImageMagick was built to read PDF at all, and whether its security policy permits it. Many servers answer yes, yes, no. The Settings screen distinguishes the three by trying a real PDF rather than by asking whether the class exists.

If nothing works

The Settings screen says what is missing and what to ask your host for. Until then, merge the files with whatever you already use on the desktop and upload the result as a normal file on the Upload step.

Originals are never deleted by a merge. They are marked Combined and kept, so the batch still records where the merged file came from.