dellus wrote

After you have converted the pdf to a dxf you first have to check what scale it is. So measure a distance in the drawing, preferably a long one, and of which you know the real life value, maybe from a dimension line. Use the tool Measure/Info - Distance Point to Point for this.
If necessary then calculate the factor to scale the drawing to the right size. Use Tools - Modify - Scale for this, select the whole drawing.

With Tools - Measure/Info - Polygonal Area you can now ask for the area of something by clicking with the mouse on corner after corner of the object until you hit the first point again (closed shape). The result is shown in the command line.
If you just need the length of a single or several lines, just select them all and at the bottom of the screen you are given the "Selected Total Length".
 
An other method to get an area is to create a hatch. You need a closed outline first, which you can draw on an extra new layer with the original drawing as guide underneath. When the hatch is created the area is reported in the command line. The advantage is you have better control on what you are doing.
But if arcs are part of the outline, due to a bug the calculated area may be wrong. I think this is fixed in the latest development version.

You can provide a sample file of a dxf converted from pdf and maybe also the original pdf. What software do you use to convert? What LibreCAD version?