Are my PDFs uploaded to a server?

No. The files are read and merged entirely inside your browser. Closing the page discards them.

Can I merge only some pages of a file?

Yes. Type a page selection next to the file, such as 1-3, 7 or 5- for page 5 to the end. The pages appear in the order you list them.

Why does the merged file not open or show a padlock?

Encrypted sources are rejected before merging. If a source PDF has an open password or print/copy restrictions, remove them with the PDF Unlock tool and merge again.

Does merging reduce quality?

No. Pages are copied object by object; images and fonts are not re-encoded, so the merged file looks exactly like the originals.

Merge PDF Files – combine PDFs in any order, free and private

How to use it

Drop two or more PDF files onto the box, or click “Choose files”. Each file shows its page count once it has been read.
Drag the files into the order you want, or use the arrow buttons. The number on the left is the position in the merged file.
Optionally type a page selection next to a file (for example 2-5 or 1, 3, 8-) to include only those pages.
Name the output file and press “Merge PDFs”. The result shows the page count and size with a download button.

Example

Combining a 12-page proposal, a 2-page cover letter and a 30-page appendix: put the cover letter first, the proposal second, then the appendix with the selection 1-10. The merged PDF has 2 + 12 + 10 = 24 pages, and its size is roughly the sum of the three sources because pages are copied, not re-rendered.

How the merge works

The tool opens each PDF with pdf-lib, copies the selected pages into a new document (copyPages), and saves the result. Copying a page brings its content stream, fonts, images and annotations along, so what you see in the source is what you get in the merged file. Shared resources are not de-duplicated between files, which is why the output is about as large as the inputs combined – run it through the PDF Compressor if size matters.

Limits

Encrypted PDFs (open password or usage restrictions) are refused; use the PDF Unlock tool first.
Bookmarks, form fields and document-level JavaScript of the sources are not carried over; page content is.
Very large files are limited by your device’s memory – several hundred MB in total is usually fine on a desktop browser.

Sources: pdf-lib documentation – copyPages.

Questions

Are my PDFs uploaded to a server?: No. The files are read and merged entirely inside your browser. Closing the page discards them.
Can I merge only some pages of a file?: Yes. Type a page selection next to the file, such as 1-3, 7 or 5- for page 5 to the end. The pages appear in the order you list them.
Why does the merged file not open or show a padlock?: Encrypted sources are rejected before merging. If a source PDF has an open password or print/copy restrictions, remove them with the PDF Unlock tool and merge again.
Does merging reduce quality?: No. Pages are copied object by object; images and fonts are not re-encoded, so the merged file looks exactly like the originals.