How to merge PDF files
Putting two PDFs together sounds like putting one file after another. Underneath it is closer to building a new document out of the pages the two files already contain.
A PDF is pages plus the structures that hold them
Every page in a PDF is its own object: a content stream, the fonts and images it refers to, its own size and its own rotation. The document keeps a list saying which order those pages come in.
Merging means creating a new document and copying pages into it. Nothing is redrawn. Text stays selectable text, vector drawings stay vectors, embedded fonts come along with the pages that use them.
This is why a merge does not need to make everything match. An A4 page and a US Letter page can sit next to each other in the result, and a landscape page stays landscape. One PDF holding pages of different sizes is ordinary, not a defect — see merging PDFs with different page sizes.
The order is yours before you save
The order files were added in is rarely the order the finished document should read in. A cover from one file, a report from another, an appendix from a third — the useful sequence usually interleaves them.
So the sequence is something you set, not something the tool decides for you. Pages can be moved, turned the right way up, copied when you need one twice, or dropped when they do not belong. None of that touches the files you opened; it describes the document you are about to create. Reordering pages before merging goes into that in more detail.
What the result is
One new PDF. Its pages are the pages you chose, in the order you put them, at their own sizes and rotations. The source files are still on your disk exactly as they were — merging reads them and writes something new beside them.
Because the pages are copied rather than re-rendered, the result is not a picture of your documents. It is your documents. Whether merging costs image quality covers that question on its own.
Doing it in Merge PDF
- Add your PDFs. They are opened in the browser and are not uploaded.
- Switch between Files and Pages to see the documents or the whole page sequence.
- Drag pages into the order you want; Rotate left / Rotate right, Duplicate and Delete work on a single page.
- Merge PDF writes the new file and downloads it.