I'll add some more information:
ArcGIS 10.6.1
Windows 7 Pro 64-bit
HP DesignJet T1120 (44-inch with a 42-inch roll installed)
Adobe Reader 11.0.23
One line and one polygon shapefile, each displaying extra lines
Geometry check showed four null geometries in the line layer and no problems in the polygon layer
Copied line data to a FGDB, still produced extra lines
All ArcGIS PDF exports at 300dpi
Data frame in ArcGIS left a 1/4-inch margin to the page edge
Unless otherwise stated, PDF dialog used "Actual Size"
Unless otherwise stated, ArcGIS paper size = printer selected paper size
30x42 map at 1:126,720 scale produced extra lines
Tried the above printer settings (Advanced > Max Application Resolution) changed from Default to 600dpi
==> same problem
Changed setting to 300dpi
==> no problem
Changed back to Default. Set custom scale in PDF print dialog. Anything below 98.1% did not have extra lines.
Shrinking to smaller paper size (using PDF "Fit" option) did not have extra lines.
Creating a PDF with a larger scale (1:100,000 and 1:80,000) in ArcGIS still had extra lines (but different lines).
Changed ArcGIS export dpi (300, 200, 100) all had extra lines (looked like same lines)
19x42 map at 1:126,720 scale produced extra lines
Export at a 1:100,000 scale - still extra lines but they are different
Set custom scale in PDF print dialog. Anything below 92.9% did not have extra lines.
24x30, 1:126,720 did not have extra lines
16x18, 1:126:720 did not have extra lines
30x41, 1:126,720 had extra lines
29x41, 1:126,720 printed on 42x30 paper, Actual size, had extra lines
29x41, 1:126,720 printed on 41x29 paper, Actual size, had extra lines
24x36, 1:110,984 did not have extra lines
24x36, 1:126,720 did not have extra lines
26x39, 1:126,720 did not have extra lines
28x40, 1:126,720 had extra lines
28x40, 1:126,720 printed on 30x40 paper, Actual size, had extra lines
It looks like data frame size, paper size, and scale might play a role, but I cannot spend any more time to narrow it down.
