The problem is that Hugin does not offer undistorted projection, suitable for scanned documents.
Thy this:
00. Before all, note down the dimensions in pixels of each partial image.
01. Load all images to Hugin
02. Set the projection to rectilinear
03. Go to Panorama Editor, Control Points tab
04. Open the same images in both view panels
05. Add two horizontal and two vertical lines. Click near the top left in the first panel and near the top right in the 2nd panel.
06. Now you have the first horizontal line. Correct the numeric values to x=0, y=0 and x=0, y=image width.
07. Do the same for the 2nd horizontal line at the bottom and two vertical lines. Then repeat the steps for all images.
08. Go to Photos tab, create control points using the Hugin CPFind.
09. Go back to the main window, save the project. This is important is something goes wrong, you can always go back (you can use Undo too, but saving is safer)
10. Back to the Editor. There are several metods of optimizing the geometrics. Try the first one (positions, incremental). If it looks not perfectly, undo and try another method.
11. You can save every method as a panorama, then compare the images. I recommend this to compare the images in full screen viewer.
11. Optimizing photometrics is optional, usable only when the images were scanned with auto exposition, auto fix, auto contrast etc.
This way I stitched some maps, but it took a lot of time and work. Easier to go to a printing service, they have A0 scanners there.