Hey Tom, thanks for the bug report! Are you totally certain the bug does not exist with type 42 sans-serif fonts? Because I also have a weird letter spacing with those fonts, as well as with the serif ones. For example,

import matplotlib as mpl
import matplotlib.pyplot as plt

plt.style.use('default')

for fam, ft in [('sans', 3), ('sans', 42), ('serif', 3), ('serif', 42)]:

    plt.rcParams['pdf.fonttype'] = ft  # in {3, 42}
    plt.rcParams['font.family'] = fam  # in {'serif', 'sans'}
    plt.rcParams['font.size'] = 20

    fig_label = "{fam}_T{ft}".format(fam=plt.rcParams['font.family'][0],
                                     ft=plt.rcParams['pdf.fonttype'])
    fig, ax = plt.subplots(num=fig_label)
    ax.plot([0, 1])
    ax.set_xlabel("mpl " + mpl.__version__, fontsize=10)
    ax.set_title("This is the title")

    fig.savefig(fig.get_label() + ".pdf")

produces weird outputs with both type 42 font families
sans_T42.pdf
serif_T42.pdf
while the outputs are OK with both type 3 font families:
sans_T3.pdf
serif_T3.pdf

Setup:
matplotlib from master ('2.0.0rc2.post3092+g2374c9b')
CentOS 7 (Linux 3.10.0-327.3.1.el7.x86_64)
Python 2.7.11 (from Continuum Analytics, Inc.)

Indeed you are right, the weird spacing happens also with sans-serif font, I overlooked this test.

I wonder if the weird spacing with type 42 fonts isn't a bigger issue than first thought. Running

import matplotlib as mpl
import matplotlib.pyplot as plt


def plot_font_grade(family, fonttype):
    plt.style.use('default')  # just to be sure

    plt.rcParams['pdf.fonttype'] = fonttype  # in {3, 42}
    plt.rcParams['font.family'] = family  # in {'serif', 'sans'}

    fig_label = "{fam}_T{ft}_grade".format(fam=plt.rcParams['font.family'][0],
                                           ft=plt.rcParams['pdf.fonttype'])
    fig, ax = plt.subplots(num=fig_label, figsize=(6.4, 12.4))

    # Cosmeticks & version info
    ax.set_xlabel("mpl " + mpl.__version__, fontsize=10)
    ax.xaxis.set_major_locator(plt.NullLocator())
    ax.yaxis.set_major_locator(plt.NullLocator())
    ax.set_ylim(-10, 600)

    # Plot text with bigger and bigger size
    for i, fs in enumerate(range(4, 50, 2)):
        ax.text(0.1, i*30, "{fs} pt font size".format(fs=fs), fontsize=fs)

    fig.savefig(fig.get_label() + ".pdf")
    plt.close(fig)


if __name__ == "__main__":

    for fam, ft in [('sans', 3), ('sans', 42), ('serif', 3), ('serif', 42)]:
        plot_font_grade(fam, ft)

results in :
type 3 fonts (OK)
sans_T3_grade.pdf
serif_T3_grade.pdf
vs type 42 fonts (does not seem OK : there are huge spaces between each character)
sans_T42_grade.pdf
serif_T42_grade.pdf
whatever the font size is…

Bisect:

cdea77c009c0c85640334825fdedb22bb4543374 is the first bad commit
commit cdea77c009c0c85640334825fdedb22bb4543374
Author: Thomas A Caswell <tcaswell@gmail.com>
Date:   Wed Oct 28 20:38:47 2015 -0400

    Merge pull request #5295 from mdboom/too-many-open-files
    
    MNT: Reduce number of font file handles opened

:100644 100644 6ea8d7a5d515fd568171cd3678ce7c8c5a73bde1 96dbae42b71737e0fd7b113086b408caa7834cbd M	INSTALL
:040000 040000 d6e17bd787602c5b461e97ec363e15144e65d73a 3d14da43cdd5bbd8c1998c923d2a0065af9d757e M	lib
:100644 100644 e294363bce578be093db779a80782c4cd905afb2 e2a6393bc4062d0408dc82afa0f2d59a6341f87a M	setup.py
:100755 100755 dd872fdc7ebdcf5ff761c84e2679d7299354dfb1 4a31bd8f049b5ed9444c9505cd69a475e205388b M	setupext.py

Seems innocuous from the title...

Editing matplotlib/font_manager.py and replacing get_font with:

def get_font(*args, **kwargs):
    return ft2font.FT2Font(*args, **kwargs)

fixes the problem, so indeed the cache is not quite right.

This fixes the test case too:

diff --git a/lib/matplotlib/backends/backend_pdf.py b/lib/matplotlib/backends/backend_pdf.py
index a5e6253ca..c293fea7c 100644
--- a/lib/matplotlib/backends/backend_pdf.py
+++ b/lib/matplotlib/backends/backend_pdf.py
@@ -41,6 +41,7 @@ from matplotlib.font_manager import findfont, is_opentype_cff_font, get_font
 from matplotlib.afm import AFM
 import matplotlib.type1font as type1font
 import matplotlib.dviread as dviread
+from matplotlib.ft2font import FT2Font
 from matplotlib.ft2font import (FIXED_WIDTH, ITALIC, LOAD_NO_SCALE,
                                 LOAD_NO_HINTING, KERNING_UNFITTED)
 from matplotlib.mathtext import MathTextParser
@@ -853,7 +854,7 @@ end"""
     def embedTTF(self, filename, characters):
         """Embed the TTF font from the named file into the document."""
 
-        font = get_font(filename)
+        font = FT2Font(filename)
         fonttype = rcParams['pdf.fonttype']
 
         def cvt(length, upe=font.units_per_EM, nearest=True):

Closed by #7961

Glad to see this is fixed. I just ran into the same funny spacing, and spent 30 min tracking it back to Type 42 fonts.

I wonder if this could be considered serious enough to trigger the release of a 2.0.1 version of matplotlib as soon as possible, since this issue prevents upgrading from the 1.5 branch whenever you use matplotlib to prepare illustrations that need to obey some editorial flow (being in PDF, using certain fonts).

For what it's worth, I like to use type 42 fonts, but I can handle type 3 fonts. All the fonts I use come in a type 42 and type 3 version. Type 3 fonts are not editable if you import a pdf into Adobe Illustrator (or similar vector graphics programs), while type 42 fonts are editable. Matplotlib, however, creates a separate text box for every individual letter when type 42 fonts are employed. This makes it a pain to edit any word longer than one letter, so type 3 fonts are not much of a downgrade. I thought I remembered a proposal to fix matplotlib's breaking up a word into multiple text boxes, but I cannot find it anywhere.

The behavior is switchable. Use the pdf.fonttype rcParam.

I'm on a linux machine (Ubuntu 16.10) where matplotlib seems to invariably pick fonts (or the font version) that show the issue. As a matter of fact, matplotlib 2.0 seems to have other font nuisances too here, like picking by default a sans serif font for math expressions in $...$. It is unclear to me how to assure that matplotlib prefers a type 3 font when the type 42 is also present.

@mdboom Last I checked, pdf.fonttype only allows you to switch between type 3 and type 42. With type 3, the text is just outlined, making it totally un-editable. With type 42 you get text that is editable, but words are broken into individual characters. Having a separate text box for each character is only slightly better than un-editable text. If you are not using a fixed width font, you can't even just edit one character at a time to change a word. You have to delete all but the first text box and start from there.

@WeatherGod I had not considered the file size issue with including the entire font package with the pdf. I like the idea of making it a rcParam, though.

The release is very soon.