[SalesForce] Community user cannot download file

Make sure that your guest site users has sharing record access to the entity the file is being shared with. I did a POC with a visualforce page exposed to public site and the guest user was able to create an account and upload a file to it without error.

Apex Controller

public class UploadDocumentDemoController {

// placeholder to upload files from visualforce page with
public Attachment file { get; set; }

public UploadDocumentDemoController() {
    this.file = new Attachment();
}

public void save() {

    System.debug( 'Saving document' );

    System.debug( file );

    // for proof of concept, will create one account for testing purposes
    // and each time we submit the form to upload file we don't create more and more accounts
    ID accountId = null;
    List<Account> accounts = [ SELECT id FROM Account WHERE name = :UserInfo.getUserId() LIMIT 1 ];
    if ( accounts.size() == 0 ) {
        Account acct = new Account(
            name = UserInfo.getUserId()
        );
        insert acct;
        accountId = acct.id;
    } else {
        accountId = accounts[0].id;
    }

    // create the chatter file
    ContentVersion cv = new ContentVersion(
        versionData = file.body,
        title = file.name,
        pathOnClient = '/' + file.name
    );

    insert cv;

    // look up the content document id
    cv = [ SELECT id, ownerId, contentDocumentId FROM ContentVersion WHERE id = :cv.id ];

    // link the document to the account
    ContentDocumentLink cdl = new ContentDocumentLink(
        linkedEntityId = accountId,
        contentDocumentId = cv.contentDocumentId,
        shareType = 'V',
        visibility = 'AllUsers'
    );

    insert cdl;

}

}

Visualforce Page

<apex:page controller="UploadDocumentDemoController" cache="false">

<apex:form>

    <apex:pageBlock>

        <apex:pageBlockButtons>
            <apex:commandButton action="{!save}" value="Upload"/>
        </apex:pageBlockButtons>

        <apex:pageBlockSection columns="1">
            <apex:inputFile value="{!file.body}" fileName="{!file.name}"/>
        </apex:pageBlockSection>

    </apex:pageBlock>

</apex:form>

Yes, the system acts inconsistently. Customer Community users cannot create contacts due to the ownership issue, but Customer Community Plus users can create contacts because the system automatically changes the contact owner to that of the portal account owner.

A before insert trigger does work to change the contact owner and allow the contact creation to occur for Customer Community license users. This code worked for me:

Trigger:

trigger ContactTriggers on Contact (before insert) {
    // Change ownership if created by Customer Community.
    ContactDomain.setOwner(Trigger.new);
}

Handler class:

public with sharing class ContactDomain {
    public static void setOwner(List<Contact> newList) {
        if (UserInfo.getUserType() == 'CSPLitePortal') {
            Id theOwnerId;

            // Use the portal account owner if active, otherwise get a random active standard user.
            User u = [SELECT Account.OwnerId, Account.Owner.IsActive FROM User WHERE Id = :UserInfo.getUserId()];
            if (u.Account.Owner.IsActive) {
                theOwnerId = u.Account.OwnerId;
            } else {
                List<User> users = [SELECT Id FROM User WHERE IsActive = true AND UserType = 'Standard' LIMIT 1];
                if (!users.isEmpty()) {
                    theOwnerId = users[0].Id;
                }
            }

            if (theOwnerId == null) {
                // Throw exception here.
            }

            for (Contact con : newList) {
                con.OwnerId = theOwnerId;
            }
        }
    }
}