Here's how to sign a document on Linux:
1. Recompile your kernel
2. Install the scanner driver
3. Download a Windows driver for your scanner that will get emulated to Linux (this actually happens for WiFi)
4. Find out there's a bug with your scanner, buy a new one
5. Find a PDF program that does one thing well, but doesn't do another things well
6. Change your distro, because the package for that particular PDF software is outdated and you really don't know how to compile the source by hand
7. Finally sign the document
8. Save the document to /tmp
9. Open your mail App
10. Send the PDF in a tar.gz
11. Recipient doesn't know WTF is a .tar.gz
12. Succumb to .zip, and send it again.
1. Why?
2. Why? SANE (Scanner Access Now Easy) takes care of scanning like CUPS does printing.
3. See 2.
4. A bug in your scanner effects
all operating systems.
5. Things that do "one thing well" is the Unix philosophy; you know the same Unix macOS is build on.
6. But you do know, you did it in step 1.
7. Just like any OS.
8. ./tmp is a sub-folder of Root, you would have to give permissions to save there. You would use your Home directory.
9. Just like any OS.
10. Why compress it at all?
11. macOS will open .tar.gz just dandy.
12. See 10. & 11.
Here's how it goes it reality
1. Scan document using Simple Scan (a front-end for SANE).
2. Open PDF ,ANY PDF program will do.
3. Print to PDF using CUPS (Common Unix Print System , yes the same macOS uses).
5. Sign document.
6. Save to your./home/USER/Documents.
7. Open your mail app, attach and send.
Either you haven't used Linux for sometime or you used it badly.