I created a simple map with a point and a buffer (3 km) in QGIS2.18 (and also 3.4) with CRS WGS84 UTM33.
With QGIS Composer I created the layout adding the scalebars (for 500 m and 3 km) and printed a map in scale 1:50000.
After exporting the map as PDF and printed it, I checked the distance on the paper map and I noticed that these are not so perfect: the measures of scalebars are ok (eg: 1 cm in the scale bar is equal to 1 cm on the ruler) but width and height of grids are little bit lower.
I also noticed that exporting the map as image (png) I have errors in scalebar (only in the 3 km one), buffer and grids, instead exporting in PDF, errors are visible only on the grids.
Pictures of measure are availabe at this link.
Did I wrong something in print setting or in the map?