Merge signed PDFs

Merging a signed PDF normally destroys its signature. This tool tells you which files are signed and offers the ways of combining them that really keep the signatures valid.

  • Free
  • No sign-up
  • Runs in your browser
merge-signed-pdf

A digital signature protects the exact bytes of the file it was made on. Merging PDFs creates a new file, so no tool can merge signed PDFs and keep those signatures valid. Below are the real options.

    100% private — your PDFs are analyzed and combined on your device and are never uploaded.

    Signature detection looks for the signature records inside each file. It does not validate the certificates, so use a PDF reader such as Adobe Acrobat to check that a signature is valid.

    How it works

    Add your PDFs

    Each file is checked for digital signatures, and you see which ones carry them.

    Choose how to combine

    Attach the signed originals inside one PDF, merge only the unsigned ones, or keep them together in a ZIP.

    Download

    Your signed originals stay untouched in the options that protect the signatures.

    Why merging breaks a digital signature

    A digital signature is not a picture of a signature. It is a cryptographic seal calculated from the exact bytes of the file when it was signed. If a single byte changes, the seal no longer matches and every PDF reader reports the signature as invalid. Merging PDFs necessarily builds a new file from the pages of the old ones, so no tool can merge signed PDFs and keep their signatures valid. Any website that promises it is either hiding that the signatures are gone or leaving the signed file out of the result.

    What actually works

    • Attach the signed originals. The tool creates one PDF with an index page and each original stored inside it as an attachment. Because each attachment is the original file, unchanged byte for byte, its signature can be extracted and validated as normal. This is the closest you can get to a single file.
    • Merge only the unsigned files. If just some documents are signed, combine the others and send the signed ones separately.
    • Keep the originals in a ZIP. Nothing is touched, which suits any recipient that accepts several files.
    • Merge anyway. The pages are combined, and the signatures stop being valid. Sometimes that is acceptable, for example when the visible content is all that matters.

    The best order of work

    If you control the documents, combine first and sign last. Merge the unsigned pages into one PDF and then sign the final file with a program such as AutoFirma or your PDF reader. That gives you a single document with one valid signature. If the signature came from someone else, such as a certificate issued to you, you cannot add pages to it afterwards.

    Check with the destination

    Some offices and portals require a single PDF, others accept several files or a ZIP, and few accept a PDF with attachments. Before choosing, check what the destination admits, since rules differ between procedures.

    What the check does and does not do

    The tool detects the signature records inside each file and tells you whether the document was changed after the last signature. It does not validate the certificate itself. To confirm that a signature is valid, open the file in a PDF reader such as Adobe Acrobat and look at its signature panel.

    If the documents are unsigned and you just want to combine them, use the plain merge PDF tool.

    Frequently asked questions

    Can I merge PDFs and keep the digital signatures valid?
    Not in a single merged document: a signature protects the exact original file, and merging creates a new one. What you can do is attach the signed originals inside one PDF, keep them together in a ZIP, or merge only the unsigned files.
    What does attaching the originals do?
    It creates a PDF with an index page and each original file stored inside as an attachment, unchanged. Anyone can extract an attachment and validate its signature as normal.
    How do I know if a PDF is signed?
    Add it to this tool. It looks for signature records inside the file and tells you how many there are. The visible mark of a signature can be a picture, so what matters is the record, not the picture.
    Does this tool check that the signature is valid?
    No. It detects that signatures exist and whether the file changed after the last one, but it does not validate the certificates. Use a PDF reader such as Adobe Acrobat for that.
    What is the best way to combine documents that still need signing?
    Merge the unsigned pages first and sign the final combined PDF once. That gives a single document with one valid signature.
    Are my files uploaded anywhere?
    No. Files are analyzed and combined in your browser and never leave your device.
    Will a portal accept a PDF with attachments?
    It depends on the portal. Many accept only plain PDFs. Check its requirements first, and if it does not accept attachments, send the signed files separately or in a ZIP.

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