My recommendation is Xournal and its actively developed fork, Xournal++. Here are the instructions.
Install (for Xournal):
sudo apt-get install xournal
For Xournal++ you can use either the stable PPA,
sudo add-apt-repository ppa:apandada1/xournalpp-stable
sudo apt update
sudo apt install xournalpp
or the flatpak,
flatpak install flathub com.github.xournalpp.xournalpp
Run xournal or xournal++, click File>Annotate PDF, choose your PDF file.
Now, go to where you need to add your signature and click Tools>Image (or the "Image" toolbar icon), then click where you want to add the image. An image selection dialog appears, select your image.
Xournal's insert image is a great addition but not polished. As soon as you add the image make sure to resize it and move it to where you want. For resize there's no ability to ensure the proportions stay the same. Just eye it. Once you are done, it is in its own layer, which you cannot change. If you don't like how it ends up delete that layer and start again.
One handy thing is that you can use ctrl-c as soon as you resize it and then ctrl-v the next time you need to insert your image. Assuming you want the same size image this will save you some time.
When you are done choose File->Export to PDF to get it back into the PDF format I assume you'll want for sending your signed doc.
Note: A downside to Xournal is the finished document looks like the fonts are converted to an image. Fonts are no longer as crisp. Still it looks better than if you printed and rescanned and is much faster. [Note: in my most recent experience it seems this problem has been solved. Maybe I just got lucky with the particular fonts used. Please leave a comment abt your experience and I'll update accordingly.] This issue seems to be fixed in Xournal++ version 1.0.20.
If you make sure the scan is 1 page per pdf, you can open en edit it with Gimp.
As long there is no text-recognition done by the scanning software, libre-office or open-office will never be able to do editing.
A simple scan gives you a image (build up by pixels) => editable by gimp
Afterwards it's possible to do text-recognition => editable by libre-office or open-office
It's also possible to make a vector drawing from the image => editable by inkscape
Best Answer
Here's my take on it from personal experience. It's a two-step process - convert the PDF into single jpegs, then import the jpegs into LibreOffice Impress. It works best if the pages in the PDF are landscape already.
1.) Make sure the package imagemagick is installed (
apt install imagemagick). Put your source PDF into a directory. Then in the terminal, navigate to that directory and run:convert filename.pdf filename.jpgThis will spit out a jpeg of each page in the PDF. Move the PDF out of the directory to make step 2 easier.
2.) Open Impress. Go to Slide menu, Slide Layout -> Blank. This will remove the "Click to add title" and stuff. Then go to Insert -> Media -> Photo Album. Click add. Navigate to the directory with all your jpegs. Select all the jpegs (if you removed the PDF from the directory in step 1, just press CTRL + A to select all of them). Click Open. The next options depend on what the pages in the original PDF looked like - if they were landscape, you can probably choose Fill Screen, but you'll have to see what works best for your case.