How to merge PDF files into one, and what to check first

To merge PDFs, you load the files, put them in order and save one new PDF that contains every page. Any decent merger does this without re-rendering, so text stays sharp and selectable. The details that catch people out are the ones around the pages: bookmarks, form fields, signatures and mixed page sizes. I built the PDF merger to run entirely in the browser, and this guide covers what it does and what to check before you send the result.

What merging actually does

A PDF is a collection of objects: each page refers to a content stream (the drawing instructions) and to the fonts and images it uses. Merging creates a new, empty document and copies each page object, with everything it refers to, from the source files into it. Nothing is converted or rasterised; a page in the merged file is the same page, at the same size and rotation, as in the original. That is why merging is fast and lossless, and why it is very different from printing several files to a new PDF, which can re-encode images and flatten interactive parts. How PDF files work: why some have text and some are pictures explains the object structure in more detail.

Merging step by step

• Choose or drop all the PDFs at once; each appears with its page count and size.
• Use the up and down arrows to set the order, and remove anything added by mistake.
• Check the summary line, which shows how many pages the merged file will have.
• Click merge and the browser saves merged.pdf.

The files are read and combined on your device and never uploaded, which matters for contracts, statements and medical records. Password-protected files are the exception: the merger reports them and asks you to remove the password first, because an encrypted file cannot be read without it.

Getting the order right

The merger orders whole files, which suits most jobs: cover letter, then CV, then certificates. If you need to move individual pages, merge first and then rearrange the result with the PDF organizer, which shows each page as a thumbnail. For a stack scanned on a single-sided scanner, with odd pages in one file and even pages in another, PDF interleave alternates them. A small habit saves time here: name the files with a leading number, such as 01-cover.pdf and 02-cv.pdf, before adding them, so they arrive close to the right order. Once the order is final, stamp continuous numbers with PDF page numbers so the reader sees page 1 to N rather than each original document's own numbering.

What does not carry over

Copying pages copies what is on the pages. Some things live at document level instead, and a page-copying merger, mine included, does not bring them across:

• Bookmarks. The outline belongs to the document, so the merged file starts without one. PDF bookmarks explained: how the outline panel works explains how they work.
• Page labels. Custom printed numbering such as i, ii, iii for front matter is stored at document level too, so viewers fall back to plain 1, 2, 3.
• Document properties such as title and author; set them again with the metadata editor if they matter.
• Interactive forms. The field widgets travel with the pages, but the document's form dictionary does not, so fields may stop behaving as fields. If two forms use the same field name, editors that do merge forms treat them as one field and typing in one fills both. Fill and flatten forms before merging.
• Digital signatures. A certificate signature covers the exact bytes of the original file, so any merged copy no longer carries a valid signature. Keep signed originals separately.

File size and mixed page sizes

A merged file is usually close to the sum of its parts. Each source brings its own fonts and images, and identical fonts from different files are not combined, so merging ten files that all use the same typeface stores ten copies of it, often as different subsets. If the result is too big to email, How to compress a PDF — what's inside a large file and what shrinks it explains the trade-offs.

Pages also keep their own sizes. Merge an A4 letter with a Letter-size invoice and a landscape spreadsheet and the merged PDF contains all three shapes, which looks untidy on screen and prints inconsistently. Run the result through the page size checker to spot this, and fit to page to bring everything to one size if needed.