In Linux, libreoffice-draw has the ability to open and validate digital signatures. Unfortunately, this functionality is broken as of today in the seahorse or gnome-keyring on LinuxMint 19.2 Tessa. This is due to the Ubuntu repository not having an updated package or a bug in seahorse.
Without digging into above, a different program named PDFStudioViewer comes to the rescue. It is a PDF viewer built using Java and has the ability to validate and verify digital signatures in a PDF.
In my case, I had a PDF document signed, but a ? icon with the signature not being verified was displayed. In most countries, the digital signature must be verified and the question mark made a tick. This can be done with Adobe Acrobat, but on Linux, there is no more a version of Adobe Acrobat reader.
To install PDF Studio Viewer, download the free version from https://www.qoppa.com/pdfstudioviewer/ look for download now and download the free Linux version for your system. I downloaded this file https://download.qoppa.com/pdfstudioviewer/PDFStudioViewer_linux64.sh
Once the file is downloaded, open a terminal window and enter the following:
$ chmod u+x PDFStudioViewer_linux64.sh <enter>
Then install it by:
Following the instructions on the screen to install.
Once you have it installed, you can open the signed PDF document, click on the yellow question mark and trust the signature.
Till such time as seahorse or gnome-keyring or Ubuntu give a fix, PDFStudioViewer does the job quite well. It is so smooth, slick and featureful, that I will replace the default Atril Document Viewer or Evince with it.
Windows 10 too does not come with a PDF viewer, except that files can be viewed using Edge. The PDFStudioViewer for Windows fills the shoes of Adobe Acrobat Reader and other PDF viewers and is a good production replacement to the Windows 10 defaults.