This has a reminder on the attached letter. vs This has a reminder to the attached letter.

Some examples from the web:

• By letter of 14 September 2006, which mentioned the said e-mail and the attached letters, Belgium stated that it had never received the letters.
• The attached letter shall be sent to Burkina Faso.
• You've got a reminder on your desk from your wife.
• We have sent a reminder on this and we think it is time to review it.
• Reference should be made to the attached letter to the Member State for a detailed description of the amendments made to the project.
• The Commission sent a reminder on 8 March 2011, and received Italy's observations on 29 April 2011.
• After the Commission had sent a reminder on 18 August 2003, the Belgian authorities replied by letter of 12 September 2003.
• The Greek authorities did not reply by the deadline set in that letter and the Commission sent them a reminder on 24 March 2009.
• Goods and/or services not eligible should be deleted in box 5 or on the attached order form.
• Let me be more specific: firstly, the agreement and the attached letter from the USA state that America will be able to transfer the information we send it to any third country it likes, with hardly any obligation to even notify us.
• Chooser - A program used to select a remote host for managing a display remotely on the attached display (gdm-host-chooser).
• The Commission invites the Advisory Committee, on the basis of Article 19(6) of the Merger Regulation, to deliver an opinion on the attached draft decision.
• (5) By letter of 26 June 2003 the Commission put a number of additional questions to the Belgian authorities. After the Commission had sent a reminder on 18 August 2003, the Belgian authorities replied by letter of 12 September 2003.
• After having sent a reminder on 16 May 2003, the Commission received from Luxembourg by fax dated 28 May 2003, registered as received on the same day, a request for an extension of the time limit for replying.
• The Member State of identification shall issue the reminder on the tenth day following that on which the return should have been submitted, and shall inform the other Member States by electronic means that a reminder has been issued.
• By letter dated 17 December 2004, the Commission asked the German authorities whether they wanted to submit further information to the notification of 7 August 2000 due to the annulment the Commission's decision and sent a reminder on 3 March 2005.
• The position of the Union within the Committee should therefore be based on the attached draft Decision,
• The position to be taken on behalf of the Union within the Committee should therefore be based on the attached draft Decision,
• Will the Commission comment on the attached article entitled 'The crucified patriarch', which appeared in the Danish newspaper Information on 12 January 2010?
• The Community shall take the following position within the ACP-EC Council of Ministers on the use of the reserve of the long-term development envelope of the 9th European Development Fund, based on the attached draft decision of the ACP-EC Council of Ministers.

Some examples from the web:

• Reference should be made to the attached letter to the Member State for a detailed description of the amendments made to the project.
• By letter of 14 September 2006, which mentioned the said e-mail and the attached letters, Belgium stated that it had never received the letters.
• Probably a reminder to relax more.
• The attached letter shall be sent to Burkina Faso.
• So I use this tattoo as a reminder to believe in something.
• It's a reminder to pick up Emily from her ballet class.
• A reminder to all candidates: Your microphones are currently on.
• Just a reminder to boil before use.
• It is a reminder to me that all things end.
• Mmm. Probably a reminder to relax more.
• A reminder to all that Origin cannot be extinguished.
• A reminder to the world there are consequences to attacking our country.
• We shall now send the Member States a reminder to ensure they respond to our request.
• Just a reminder to please make a donation to one of our charity baskets.
• A reminder to all passengers that due to engineering work no trains are currently stopping at Clapham Junction.
• It's a reminder to hold the people we love close, 'cause you never know when you could lose them.
• You know, it's events like this act as a reminder to us all how fleeting is life and opportunity.
• I am just typing a reminder to myself to shape up at school.
• The apprehension of Richard Tyler is a huge win for us, a reminder to everyone that the rule of law still prevails in this country.
• The recent violence is a reminder to us that our priority should be to reach definite agreements as soon as possible.