diff --git a/README.md b/README.md
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--- a/README.md
+++ b/README.md
@@ -26,8 +26,12 @@ Good authors' blogs or profiles are linked as well to keep following there writi
 - [linkName](url), Author: short description.
 ```
 ### Tech
+#### Tips
 - [Lessons Learned in Software Development](https://henrikwarne.com/2015/04/16/lessons-learned-in-software-development/), Henrik Warne: Tips and Lessons for software developers.
+#### Automation
 - [Discord Webhooks Guide](https://birdie0.github.io/discord-webhooks-guide/), birdie0: Comprehensive guide for discord webhook.
+#### Security
+- [Public Key Infrastructure Overview](/static/pdfs/pki-overview.pdf), Sun mircosystems.
 ---
 ### Life
 ---
diff --git a/static/pdfs/pki-overview.pdf b/static/pdfs/pki-overview.pdf
new file mode 100644
index 0000000..743bd21
Binary files /dev/null and b/static/pdfs/pki-overview.pdf differ