diff --git a/browser/locales/en-US/pdfviewer/viewer.properties b/browser/locales/en-US/pdfviewer/viewer.properties
index 25abd9275aed29155be342873677953117739dac..cdb84d05fd707c3cff08c827c6f1bfd28af6f81b 100644
--- a/browser/locales/en-US/pdfviewer/viewer.properties
+++ b/browser/locales/en-US/pdfviewer/viewer.properties
@@ -3,8 +3,14 @@ previous.title=Previous Page
 previous_label=Previous
 next.title=Next Page
 next_label=Next
+
+# LOCALIZATION NOTE (page_label, page_of):
+# These strings are concatenated to form the "Page: X of Y" string.
+# Do not translate "{{pageCount}}", it will be substituted with a number
+# representing the total number of pages.
 page_label=Page:
 page_of=of {{pageCount}}
+
 zoom_out.title=Zoom Out
 zoom_out_label=Zoom Out
 zoom_in.title=Zoom In
@@ -19,7 +25,9 @@ download_label=Download
 bookmark.title=Current view (copy or open in new window)
 bookmark_label=Current View
 
-# Side panel toolbar buttons (tooltips and alt text for images)
+# Tooltips and alt text for side panel toolbar buttons
+# (the _label strings are alt text for the buttons, the .title strings are
+# tooltips)
 toggle_slider.title=Toggle Slider
 toggle_slider_label=Toggle Slider
 outline.title=Show Document Outline
@@ -33,7 +41,11 @@ search_panel_label=Search
 no_outline=No Outline Available
 
 # Thumbnails panel item (tooltip and alt text for images)
+# LOCALIZATION NOTE (thumb_page_title): "{{page}}" will be replaced by the page
+# number.
 thumb_page_title=Page {{page}}
+# LOCALIZATION NOTE (thumb_page_canvas): "{{page}}" will be replaced by the page
+# number.
 thumb_page_canvas=Thumbnail of Page {{page}}
 
 # Search panel button title and messages
@@ -44,10 +56,18 @@ search_terms_not_found=(Not found)
 error_more_info=More Information
 error_less_info=Less Information
 error_close=Close
+# LOCALIZATION NOTE (error_build): "{{build}}" will be replaced by the PDF.JS
+# build ID.
 error_build=PDF.JS Build: {{build}}
+# LOCALIZATION NOTE (error_message): "{{message}}" will be replaced by an
+# english string describing the error.
 error_message=Message: {{message}}
+# LOCALIZATION NOTE (error_stack): "{{stack}}" will be replaced with a stack
+# trace.
 error_stack=Stack: {{stack}}
+# LOCALIZATION NOTE (error_file): "{{file}}" will be replaced with a filename
 error_file=File: {{file}}
+# LOCALIZATION NOTE (error_line): "{{line}}" will be replaced with a line number
 error_line=Line: {{line}}
 rendering_error=An error occurred while rendering the page.
 
@@ -58,10 +78,14 @@ page_scale_auto=Automatic Zoom
 page_scale_actual=Actual Size
 
 # Loading indicator messages
-loading=Loading... {{percent}}%
+# LOCALIZATION NOTE (error_line): "{{[percent}}" will be replaced with a percentage
+loading=Loading… {{percent}}%
 loading_error_indicator=Error
 loading_error=An error occurred while loading the PDF.
 
-# Misc labels and messages
+# LOCALIZATION NOTE (text_annotation_type): This is used as a tooltip.
+# "{{[type}}" will be replaced with an annotation type from a list defined in
+# the PDF spec (32000-1:2008 Table 169 – Annotation types).
+# Some common types are e.g.: "Check", "Text", "Comment", "Note"
 text_annotation_type=[{{type}} Annotation]
 request_password=PDF is protected by a password: