Questions[open-source](ubuntu)

This older post is either pointing to mostly dead software or the answers are not fully applicable.

I want to take a PDF document, stick in an image of my signature and have this be digitally signed using a certificate so that the document is secured and any changes will be picked up.

I'd like to open a document, navigate to the relevant signature page, click on the line or draw a box, enter a password and my signature should be drawn and certificate used to digitally sign the doc.

I've tried the following options and here are the problems:

• Libre Office: Difficult to sign existing PDF's, better to create pdf's with. Have to add signature image separately.

• PortableSigner: Hard to position signature but does the job

• Master PDF Editor: Works well but takes 70 dollars to prevent ugly watermark being added to PDF's

• Foxit Reader: Only adds image without any certificate signing.

Any ideas?

I know I can check the currently installed non-free software with the following command (assuming aptitude is FOSS):

aptitude search '?installed (?section(restricted) | ?section(multiverse))'

And I can remove the "restricted" and "multiverse" sources (and not add any PPA's) to keep that list empty. I'm assuming from this point, whenever I apt install something, that something is either in the allowed sources or it is not found.

Is this enough? Does this make sure all software I'm running (in Ubuntu) is open-source?

My question is very simple: how to get rid of proprietary software from the Ubuntu Software Center (USC)?

It doesn’t work to enable or disable repositories. Even with just the “universe” and “main” repositories active you can still see plenty of proprietary software. Examples include Skype, Slack, Spotiffy.

Tested: Ubuntu 18.04 and Ubuntu 16.04.

Edited: I had to edit this question down to exclude another bug that I previously reported here about how many open source apps in the USC are mislabeled as proprietary. Added that here https://askubuntu.com/questions/1052688/open-source-software-mislabeled-in-ubuntu-software-center-usc-as-proprietary

UPDATE: This question remains unanswered and I believe it is a very important topic so it needs more attention. If Ubuntu claims to be Free and Open Source then give me the freedom to remove the proprietary software from your official Software Center please. Newcomers to Ubuntu Operating System will not know how to avoid the proprietary apps from the Software Center, especially since when they (I, and all of us) install Ubuntu we may choose only the Open Source repositories, yet the Software Center is full of proprietary apps.

UPDATE 2: 33% of the software is proprietary when you visit the USC home. Video https://www.youtube.com/watch?v=PIVHQj7pfXQ&feature=youtu.be Also Krita appears as proprietary and I reported here Krita labeled as proprietary in Ubuntu Software Center

I am looking for an open source command line tool to crop PDF file just like we can do in Adobe Acrobat Pro. I have tried PdfTk, ImageMagick, PyPDF, and GhostScript—all with no success so far.

I'm looking for an open source Skype application that I can use instead of the current beta we have to install.