diff --git a/knowledge base/atom.md b/knowledge base/atom.md
index 59d2f9a..55fa46b 100644
--- a/knowledge base/atom.md	
+++ b/knowledge base/atom.md	
@@ -7,7 +7,7 @@
 Change font to `NotoSansMono Nerd Font` in Platformio Ide Terminal's settings.  
 See [Why Zsh terminal icons are not getting displayed in Atom Platformio Ide Terminal?]
 
-## Sources
+## Further readings
 
 - [Why Zsh terminal icons are not getting displayed in Atom Platformio Ide Terminal?]
 
diff --git a/knowledge base/grep the standard error stream.md b/knowledge base/grep the standard error stream.md
index 7f6ec20..2de1180 100644
--- a/knowledge base/grep the standard error stream.md	
+++ b/knowledge base/grep the standard error stream.md	
@@ -26,13 +26,10 @@ command 1>/dev/null 2> >(grep -oP "(.*)(?=pattern)")
 > Do note that **the target command of process substitution runs asynchronously**.  
 > As a consequence, `stderr` lines that get through the grep filter may not appear at the place you would expect in the rest of the output, but even on your next command prompt.
 
-## See also
-
-- Knowledge base on [grep]
-
 ## Further readings
 
 - Answer on [StackExchange] about [how to grep the standard error stream]
+- Knowledge base on [grep]
 
 [grep]: grep.md
 
diff --git a/knowledge base/grep.md b/knowledge base/grep.md
index bb854a7..4bf8609 100644
--- a/knowledge base/grep.md	
+++ b/knowledge base/grep.md	
@@ -8,7 +8,7 @@ grep 'pattern' path/to/search
 
 # recursive search
 grep -R 'pattern' path/to/search/recursively
-grep -R --exclude-dir excluded/dir 'pattern' path/to/search/recursively  # gnu grep >= 2.5.2
+grep -R --exclude-dir excluded/dir 'pattern' path/to/search/recursively   # gnu grep >= 2.5.2
 
 # show line numbers
 grep -n 'pattern' path/to/search
diff --git a/knowledge base/mount samba shares from a unix client.md b/knowledge base/mount samba shares from a unix client.md
index 241cc3f..8e2565b 100644
--- a/knowledge base/mount samba shares from a unix client.md	
+++ b/knowledge base/mount samba shares from a unix client.md	
@@ -6,7 +6,7 @@
 sudo mount -t cifs -o user=my-user //nas.local/shared_folder local_folder
 ```
 
-## Sources
+## Further readings
 
 - [Mounting samba shares from a unix client]
 
diff --git a/knowledge base/opkg.md b/knowledge base/opkg.md
index 25836d7..86afd63 100644
--- a/knowledge base/opkg.md	
+++ b/knowledge base/opkg.md	
@@ -38,7 +38,7 @@ opkg list-changed-conffiles
 opkg depends dropbear
 ```
 
-## Sources
+## Further readings
 
 - [Opkg package manager]
 
diff --git a/knowledge base/pre-commit.md b/knowledge base/pre-commit.md
index 0525511..164e2d0 100644
--- a/knowledge base/pre-commit.md	
+++ b/knowledge base/pre-commit.md	
@@ -7,8 +7,8 @@
 pre-commit sample-config > .pre-commit-config.yaml
 
 # manually run checks
-pre-commit run --all-files   # all checks
-pre-commit run ansible-lint  # ansible-lint only
+pre-commit run --all-files    # all checks
+pre-commit run ansible-lint   # ansible-lint only
 
 # automatically run checks at every commit
 pre-commit install
@@ -22,12 +22,8 @@ SKIP=flake8 git commit -m "foo"
 
 ## Further readings
 
+- Pre-commit's [website]
 - List of [supported hooks]
 
 [supported hooks]: https://pre-commit.com/hooks.html
-
-## Sources
-
-- [Website]
-
 [website]: https://pre-commit.com
diff --git a/knowledge base/rotate a pdf file.md b/knowledge base/rotate a pdf file.md
new file mode 100644
index 0000000..da73342
--- /dev/null
+++ b/knowledge base/rotate a pdf file.md	
@@ -0,0 +1,7 @@
+# Rotate a PDF file
+
+## Further readings
+
+- [pdftk]
+
+[pdftk]: pdftk.md#rotate-a-file
diff --git a/knowledge base/sponge.md b/knowledge base/sponge.md
index 45c0df3..d1a05f5 100644
--- a/knowledge base/sponge.md	
+++ b/knowledge base/sponge.md	
@@ -9,7 +9,7 @@ brew install sponge   # or moreutils
 # append file content to the source file
 cat path/to/file | sponge -a path/to/other/file
 
-# remove all lines starting with # in a file:
+# remove all lines starting with "#" in a file
 grep -v '^{{#}}' path/to/file | sponge path/to/other/file
 ```