Hello,
Using LO v6.0.2.1 on Mint 18.3 there is no problem with PDF export for, document, sheet, or selected items.
Installed v5.1.6.2 on the system and “Range” is available. Then by choosing ‘Selection’ under range it will export selected sheet items or if none selected, the active sheet.
If your “Range” is greyed out you can try resetting your user profile. See this page → LibreOffice user profile.
You may want to upgrade your version to a newer one. v5.4.x & v6.0.x are the latest. It also may be where you are getting you LO from. If it is a distro version (included with & updated through Ubuntu), it may be a distro bug. The mentioned versions tested with are from TDF (The Document Foundation).