Hoje fui contatado pela GlobalSign com uma explicação muito melhor, que acho que responde a essa pergunta:
So our PersonalSign product can be used to sign documents like Word Documents, Excel, Open/LibreOffice files, and even PDFs. The PersonalSign can also be used for e-mail signatures, e-mail encryption, and client / 2-Factor authentication.
The reason there is a separate product for PDF signatures is that while our PersonalSign product has the technical capability to sign a PDF, they will not show as trusted in Adobe Acrobat. It's not really our distinction, but a separate product that abides by Adobe's standards for signatures. Those certificates ultimately chain back to Adobe's Root Certificate so that they are trusted in Acrobat. Additionally the PDF signing certificate (in compliance with Adobe's CDS standards) are installed on a USB Token and cannot be exported from the device, they also place a timestamp using one of Adobe's servers enabling long term validity. There is also a higher level of vetting on those certificates than on our standard PersonalSign products.
So, the additional additional vetting, equipment, and compliance requirements increase the cost of that certificate.
And just to re-iterate, you CAN place a signature on a PDF with our PersonalSign certificate, it just will not show the "Blue Ribbon" that it's trusted in Adobe Acrobat. So depending on your needs for a signature, this may or may not be a viable option.