grep '\.pdf$' keeps lines ending in .pdf. There are two implementation notes worth having.
The invisible carriage return
A file with Windows line endings has \r before every \n, so no line ends with .pdf — it ends with .pdf\r. The filter returns nothing against data you can plainly see contains PDFs. This is the single most common cause of a suffix filter that "does not work". Strip \r first.
Suffix checks do not need regex
Most languages have endsWith, which is a direct comparison against the tail of the string. It is faster than a regex engine, and it cannot be confused by metacharacters in the suffix — no escaping the dot, no accidental wildcard.
Case sensitivity in file extensions
URLs may carry .PDF as readily as .pdf. Filesystems vary in whether they care; your filter should be explicit either way rather than inheriting a default.
Try it: Keep Lines Ending With on SeoWolf's Notepad.