Digital signatures
What a signature is worth, PAdES, and the five legal levels.
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How to sign a PDF with a digital certificate on a Mac
From a certificate file to a signed PDF: what a signing certificate is, how it gets into the macOS keychain, how to sign with Adobe Acrobat and with PDFsigna, and how to check the signature afterwards.
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Acrobat alternatives you buy once
Adobe no longer sells a perpetual Acrobat. What it sells instead, which PDF editors still have a one-time price, the four questions to ask before you pay — with every figure checked on the vendor’s own site in October 2026.
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A visual signature is not a digital signature
Your handwriting placed on a PDF is not an advanced electronic signature. What each one is, what it is worth, and how PDFsigna tells you before you sign.
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How to check whether a PDF signature is valid
Three free ways to validate the signature on a PDF someone sent you — the European Commission’s DSS demo, Spain’s VALIDe and Adobe Acrobat — and what valid, indeterminate, invalid and modified actually mean.
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B-B, B-T, B-LT, B-LTA: which PAdES level do you need?
The four levels of a PAdES signature in plain language: what each one adds, and when it is enough for an agreement, an invoice, or a document someone must still be able to verify in ten years.
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Why your signature needs a third party’s clock
What a timestamping authority is, why your own computer’s clock proves nothing, and why PDFsigna timestamps signatures with a qualified, free-for-personal-use authority.