There are some sample questions here.
http://flexandsalesforce.blogspot.ca/2015/08/salesforce-certified-platform-developer_19.html

Here is what I used to study and the topics i had on my exam
1. Force.com workbook:
https://resources.docs.salesforce.com/sfdc/pdf/forcecom_workbook.pdf
2. Apex workbook:
https://resources.docs.salesforce.com/sfdc/pdf/apex_workbook.pdf
3. Visualforce workbook:
https://developer.salesforce.com/docs/atlas.en-us.workbook_vf.meta/workbook_vf/
4. migration tool dev guide:
https://resources.docs.salesforce.com/sfdc/pdf/salesforce_migration_guide.pdf
5. development lifecycle guide:
https://resources.docs.salesforce.com/sfdc/pdf/salesforce_development_lifecycle.pdf

Brush up on the following topics:

Rollup field, formula field, validation rule
workflow vs Visual workflow vs process builder vs trigger
outbound message
order of execution
running test classes, test methods.
dev console tabs
multi select picklist
custom ctrl, standard ctrl, extension, standardSetCtrl
unmanaged package vs managed package
change set, deployment
different types of sandboxes
governer limits
limits object and methods
lightning benefits, components, controllers
before vs after trigger
trigger.new limitations
types of exception in apex
best practices for apex
when to use declarative vs custom code
callouts
@future methods
force.com ide funcitonality
tools to run tests, see logs.
types of logs and where to set the filters in their new debug UI


Good luck. 

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