How to Combine PDF Files, and What Survives the Merge
How to merge PDF files into one: file order, what happens to bookmarks, links and forms, password-protected PDFs, and merging on Windows, Mac and iPhone.
By Udit Agarwal · Updated October 18, 2026
Merging PDF files copies every page of each file, one file after another and in the order you choose, into a single new document. The pages are copied as they are, so text stays selectable and images keep their quality. What a merge can leave behind is everything that belongs to the whole document rather than to a page: the bookmarks, the fillable form, the title and author, and custom page numbering.
A PDF is built from objects. Each page is an object that points to its content: the drawing instructions for its text and graphics, plus the fonts and images it uses. Above the pages sits the document catalog, which the PDF standard, ISO 32000, makes the root of the file. It holds the list of pages and the parts that describe the whole document, such as the bookmark outline, the interactive form, the page labels and the targets of internal links. The title and author are stored at document level too.
A merge starts a new document and copies the page objects from each file into its page list. Because the pages are copied rather than redrawn, nothing on them changes. Everything that hangs off the old catalogs has to be rebuilt by the merging program. Some programs rebuild bookmarks and forms; others, ArrayKit's merge tool among them, keep only the pages.
The order of the files is the order of the pages, so settle it before you merge. If you add several files at once, the order they arrive in depends on your browser and system, so check the list. Numbering the file names (01-cover.pdf, 02-report.pdf) keeps them in order whenever they are sorted by name.
Three situations need more than reordering whole files:
Pages keep their own size and orientation, so merging A4 and US Letter files gives a document with mixed page sizes. If you need one size throughout, Resize PDF rescales every page to A4, Letter or another standard size.
Anything that lives on a page travels with it. Anything that belongs to the document as a whole needs rebuilding, and a tool that copies only pages leaves it behind. This is what happens when you merge with ArrayKit:
| Part of the PDF | After merging |
|---|---|
| Text, images and drawings | Kept exactly; text stays selectable |
| Page size and rotation | Kept for each page |
| Comments, highlights and sticky notes | Kept |
| Links to websites | Kept |
| Links to other pages in the same file | Kept, and they still jump to the right page |
| Bookmarks | Lost |
| Fillable form fields | Still show their values, but may no longer be fillable |
| Title, author, subject and keywords | Lost; the file gets fresh document information |
| Page labels (i, ii, iii, then 1, 2, 3) | Lost; pages are numbered from 1 |
| Password protection | Not carried over to the merged file |
| Document-level attachments, language and accessibility tags | Lost |
Most losses have a fix. The PDF Bookmark Editor rebuilds bookmarks in the merged file: one per page, from the headings in the text, or from a pasted outline with a title and page number on each line. If the values typed into a form matter, run each file through Flatten PDF before merging; it paints the values onto the page, where they can no longer be lost or changed. One more trap: in a PDF form, fields with the same name share one value, so a tool that keeps forms can leave two files' "Name" fields sharing one answer.
A merged PDF weighs roughly what its inputs add up to, because every page brings its own content, fonts and images. With ArrayKit it can come out somewhat smaller, because the merge stores the file's internal structure compactly and leaves out data that no page uses, such as leftovers from earlier edits. Three small PDFs of 30, 24 and 35 KB, 89 KB together, merged into a 77 KB file.
A merge does not shrink a file by sharing, though. If five files each embed the same font or the same logo, the merged file holds five copies, because each file's copy is a separate object. To bring a large result down, compress it; why PDFs get big explains which methods keep text selectable and which turn pages into pictures. If the file has to fit an attachment limit, splitting it into parts under the limit is the other route.
A PDF can carry two passwords. The open password, also called the user password, is needed to view the file at all. The permissions password, or owner password, lets anyone open the file but asks PDF readers to block printing, copying or editing. Both work by encrypting the file's contents, so a file that opens without any password can still be encrypted.
A merge tool has to decrypt a file before it can copy its pages. ArrayKit's merge tool does not decrypt, so it marks a protected file "Password-protected" and leaves it out rather than adding blank pages. PDF Inspector tells you whether a file is encrypted. If it opens without a password and only restricts printing or copying, Remove PDF Restrictions saves a clean, unencrypted copy you can merge; if it needs an open password, Decrypt PDF removes the protection once you enter it. If a file needs an open password you were never given, ask the sender for an unprotected copy.
The merged file has no password, whatever its inputs had. If it needs protecting, add a password after merging.
Built-in apps can do the job on some systems:
A browser-based tool works the same way on all of these, and on Android phones and Chromebooks. What to check is where your files go: many online PDF services upload them to their servers, while a tool that runs in the browser processes them on your device.
| Problem | Likely cause | What to do |
|---|---|---|
| A file shows "Could not read" | The file is damaged or is not really a PDF | Re-save it from the app that made it, or try Repair PDF |
| A file shows "Password-protected" | That input is encrypted | Save a clean copy with Remove PDF Restrictions, or with Decrypt PDF if it asks for a password, then add that copy |
| A photo or Word file is ignored | Only PDFs can be merged | Convert it first, for example with Image to PDF |
| Bookmarks or form fields are gone | The merge copied pages only | Rebuild bookmarks; flatten forms before merging |
| The result is too large to email | Every input's images and fonts add up | Compress it, or split it into parts |
| The browser slows down | Very large files fill the device's memory | Merge in batches, then merge the results |
Merge PDFs joins whole files. Drop in several PDFs at once, or add more later; anything that is not a PDF is skipped with a warning. Each file appears in a list with its name, size and page count, and arrows move it up or down, with buttons to remove one file or clear the list. Merge & download saves a single file, merged.pdf, holding every page of every readable file in list order. There is no fixed limit on the number or size of files beyond your device's memory. The PDFs are read and merged in your browser and are not uploaded.
Its limits are the ones described above. It takes whole files, so choose pages with Split PDF first. It keeps no bookmarks, document information or page labels, and form fields may stop being fillable. It cannot decrypt protected files, so they are marked "Password-protected" and left out, and a file it cannot read at all is marked "Could not read (possibly encrypted)" and left out too.
How do I merge PDF files into one? Add the files to a merge tool, put them in the order you want and merge. On a Mac you can also use Preview, and on an iPhone the Create PDF command in the Files app. Windows has no built-in option, so use a browser-based tool or a PDF editor.
Does merging PDFs reduce the quality? No. A merge copies pages without redrawing them, so text, images and drawings stay exactly as they were. Quality drops only if you later compress the merged file with a method that lowers image resolution.
Why did my bookmarks disappear after merging? Bookmarks belong to the document, not to its pages, and a tool that copies only pages leaves them behind. Rebuild them in the merged file with a bookmark editor, pointing each one at the page where an original file now starts.
Can I merge password-protected PDFs? Only after removing the protection. ArrayKit marks a protected file and leaves it out of the merge, so make an unprotected copy first: Remove PDF Restrictions for a file that opens without a password, or Decrypt PDF with the open password for one that does not. The merged file itself has no password.
How do I merge only some pages of a PDF? Extract the pages you want into a new file with Split PDF, then merge that file with the others. You can also merge everything and delete pages afterwards with Organize PDF.
Is it safe to merge PDFs online? It depends on where the work happens. Many online services upload your files to their servers. ArrayKit's merge tool runs in your browser, so the files stay on your device.