First of all, you have to determine exactly how is this plan drawn. You mentioned 1:1 scale.
But this can be 1 unit = 1 mm or 1 cm or even 1 Meter. There are ways to determine exactly which one this is.
Since this is an Architectural floor plan, you can use the DIST or DIM command to measure a door width.


Since they both show 0.75 which would be a typical size residential door if it's 0.75 Meter and not cm or mm.
Then this proves that this drawing is drawn as 1 unit = 1 Meter
Next, run the UNITS command and make the following adjustments:

Under Length:
The Type is as you've set properly as Decimal for a metric drawing.
The Precision though should not be 0.0000. There's no reason for a 4th place precision since there should be no measurements smaller than a mm. So I changed this setting to 0.000.
The Insertion Scale should not be left as Inches since this is a metric drawing and in Meters. This setting is used when the current drawing is referenced or inserted into another drawing. AutoCAD will use this value to scale the dwg up/dn to match with that other drawing's drawing unit.
Then I noticed that your DIASTASEIS dimension style is missing some settings like extension lines so I made the following adjustments using the DDIM command which I typically implement with drawing Architectural floor plans:
Under Lines tab uncheck Suppress Ext line 1 & Ext line 2

Under Fit tab:
Fit options: select Always keep text between ext lines
Fine tuning: check Draw dim line between ext lines

Next go to Layout1 to setup the 1:50 scale plot:
Since AutoCAD typically creates a Paperspace vport for you already in the default Layout1 you can use this to see how large of a Vport you'll need to accommodate a 1:50 scale plan drawn at 1 unit = 1 Meter.
By default, AutoCAD's Layouts are also set up to use paper sheet sizes in mm.
So you'll need to convert from Meter in Modelspace to mm in Paperspace.
The typical method to properly change the zoom scale factor of a Paperspace Vport is to enter into the Vport by double clicking inside it and then use the ZOOM command ##XP factor. Since the scale factor you want is 1:50 then we're looking at 1/0.050XP = 20XP. Since this 20 = 1 unit scale is not on the typical scale list then I used the ScalelistEdit command to add this in:

Though not required, it's easier to be able to select 20:1 from the scale list instead of having to enter 1/0.050 or 20.
Now you can select the Paperspace Vport and on the Properties palette select under Standard scale's dropdown list 20:1

After expanding the size of the Paperspace Vport so the entire plan can be seen, next is to determine which paper size would work ie: A4, A3 or A2
Run the PAGESETUP command and select Modify the current Layout1
This Page Setup window looks very similar to the PLOT command window.
For printer with DWG To PDF.pc3 selected, now you can try various Paper sizes to see which will fit in the Plot area setup by default as Layout with Plot scale 1:1 mm.

What I ended up selecting for Paper size is A2. Since the top rows of dimensions don't fit I created an additional Paperspace Vport on top collapsing the space between the top dimensions and the floor plan.
I also created another Paperspace Vport on the right to squeeze in the sign with the info on the lower right.
Now when the A2 size paper is folded once = A3 size paper and then folded again = A4 size the the sign with the info will still remain in the lower right.

Lastly I placed the Paperspace Vports on a Non Plot Layer
see attached wg.
As you've heard from others your design floor plan at 1:50 will not fit on A3 size paper.