Strange rendering of graphics exported as .pdf

Question

Bug introduced in 10.4.1 or earlier and persisting through 12.0
^{Bug isn't present in version 8.0.4}

---

I am trying to export a graphic generated by the following code to a .pdf file.

c := 1;
ctoP = 0.80;
P := c/ctoP;
s0 := 0.0;
sf := 1.5;
\[Theta]0 := Pi/180*45;
theta = 
 DSolve[{\[Theta]'[s] == Sqrt[
 2.0 (c + P Cos[\[Theta][s]])], \[Theta][
 0] == \[Theta]0}, \[Theta], s];

elastica = 
  NDSolve[{x'[s] == Cos[\[Theta][s]], y'[s] == Sin[\[Theta][s]], 
  x[0] == 0, y[0] == 0} /. theta, {x, y}, {s, s0, sf}];

deformed = 
 ParametricPlot[{x[s], y[s]} /. elastica, {s, s0, sf}, 
  PlotPoints -> 500, Frame -> False, Axes -> False,

  FrameStyle -> Black,

  PlotPoints -> 200,

  PlotStyle -> {Black, 
    Thickness[0.003]},

  PlotRange -> {{-8, +8}, {-2, 4}},

  ImageSize -> Large
  ]


Export["figure.pdf", deformed, ImageResolution -> 1500]

The file "figure.pdf" obtained has the following unwanted features (highlighted using red ovals) on it:

Interestingly enough, these features do not appear if the file is exported as a .png or .jpeg.

I've also tried exporting the graphic as an .eps file, but the problem still remains.

I need the file in .pdf format because I don't want my figures to be rasterized.

How can I get rid of these artifacts on the pdf and obtain a nice smooth curve?

Answer 1

This looks like a bug in PDF export. I do reproduce it with version 10.4.1 and 11.1.1 but not with version 8.0.4 on Windows 7 x64.

In order to ensure that it is not due to the other functions used I saved the deformed plot generated by Mathematica 8.0.4 and then imported it into more recent versions for further exporting as PDF:

(* in Mathematica 8.0.4 *)

deformed >> deformed.m

(* in more recent versions *)

Export["figure.pdf", << deformed.m] // SystemOpen

Here is how the files exported from versions 10.4.1 and 11.1.1 are rendered by Adobe Acrobat 11:

And here is how the file exported from Mathematica 8.0.4 is rendered: