diff --git a/src/calibre/ebooks/conversion/plugins/pdf_output.py b/src/calibre/ebooks/conversion/plugins/pdf_output.py
index 37cb9529a5..d27a9da214 100644
--- a/src/calibre/ebooks/conversion/plugins/pdf_output.py
+++ b/src/calibre/ebooks/conversion/plugins/pdf_output.py
@@ -69,10 +69,10 @@ class PDFOutput(OutputFormatPlugin):
                 'The font family used to render monospace fonts')),
         OptionRecommendation(name='pdf_default_font_size',
             recommended_value=20, help=_(
-                'The default font size')),
+                'The default font size (in pixels)')),
         OptionRecommendation(name='pdf_mono_font_size',
             recommended_value=16, help=_(
-                'The default font size for monospaced text')),
+                'The default font size for monospaced text (in pixels)')),
         OptionRecommendation(name='pdf_hyphenate', recommended_value=False,
             help=_('Break long words at the end of lines. This can give the text at the right margin a more even appearance.'
                    ' Note that depending on the fonts used this option can break the copying of text from the PDF file.')),
diff --git a/src/calibre/ebooks/pdf/html_writer.py b/src/calibre/ebooks/pdf/html_writer.py
index a1c6901466..d3bd5b0d5e 100644
--- a/src/calibre/ebooks/pdf/html_writer.py
+++ b/src/calibre/ebooks/pdf/html_writer.py
@@ -152,8 +152,8 @@ class Renderer(QWebEnginePage):
         self.wait_for_title = None
         s = self.settings()
         s.setAttribute(s.JavascriptEnabled, True)
-        s.setFontSize(s.DefaultFontSize, opts.pdf_default_font_size)
-        s.setFontSize(s.DefaultFixedFontSize, opts.pdf_mono_font_size)
+        s.setFontSize(s.DefaultFontSize, int(opts.pdf_default_font_size))
+        s.setFontSize(s.DefaultFixedFontSize, int(opts.pdf_mono_font_size))
         s.setFontSize(s.MinimumLogicalFontSize, 8)
         s.setFontSize(s.MinimumFontSize, 8)
         std = {
diff --git a/src/calibre/gui2/tweak_book/preview.py b/src/calibre/gui2/tweak_book/preview.py
index dac64efdc4..8bcce528ac 100644
--- a/src/calibre/gui2/tweak_book/preview.py
+++ b/src/calibre/gui2/tweak_book/preview.py
@@ -763,10 +763,10 @@ class Preview(QWidget):
 
     def apply_settings(self):
         s = self.view.settings()
-        s.setFontSize(QWebEngineSettings.FontSize.DefaultFontSize, tprefs['preview_base_font_size'])
-        s.setFontSize(QWebEngineSettings.FontSize.DefaultFixedFontSize, tprefs['preview_mono_font_size'])
-        s.setFontSize(QWebEngineSettings.FontSize.MinimumLogicalFontSize, tprefs['preview_minimum_font_size'])
-        s.setFontSize(QWebEngineSettings.FontSize.MinimumFontSize, tprefs['preview_minimum_font_size'])
+        s.setFontSize(QWebEngineSettings.FontSize.DefaultFontSize, int(tprefs['preview_base_font_size']))
+        s.setFontSize(QWebEngineSettings.FontSize.DefaultFixedFontSize, int(tprefs['preview_mono_font_size']))
+        s.setFontSize(QWebEngineSettings.FontSize.MinimumLogicalFontSize, int(tprefs['preview_minimum_font_size']))
+        s.setFontSize(QWebEngineSettings.FontSize.MinimumFontSize, int(tprefs['preview_minimum_font_size']))
         sf, ssf, mf = tprefs['engine_preview_serif_family'], tprefs['engine_preview_sans_family'], tprefs['engine_preview_mono_family']
         if sf:
             s.setFontFamily(QWebEngineSettings.FontFamily.SerifFont, sf)
diff --git a/src/calibre/gui2/viewer/web_view.py b/src/calibre/gui2/viewer/web_view.py
index bd2c10dcea..f6edd0e2f6 100644
--- a/src/calibre/gui2/viewer/web_view.py
+++ b/src/calibre/gui2/viewer/web_view.py
@@ -322,10 +322,10 @@ def apply_font_settings(page_or_view):
     if mfs is None:
         s.resetFontSize(QWebEngineSettings.FontSize.MinimumFontSize)
     else:
-        s.setFontSize(QWebEngineSettings.FontSize.MinimumFontSize, mfs)
+        s.setFontSize(QWebEngineSettings.FontSize.MinimumFontSize, int(mfs))
     bfs = sd.get('base_font_size')
     if bfs is not None:
-        s.setFontSize(QWebEngineSettings.FontSize.DefaultFontSize, bfs)
+        s.setFontSize(QWebEngineSettings.FontSize.DefaultFontSize, int(bfs))
         s.setFontSize(QWebEngineSettings.FontSize.DefaultFixedFontSize, int(bfs * 13 / 16))
 
     font_size_changed = (old_minimum, old_base, old_fixed_base) != (